I planned this thread a couple of days ago but procrastinated on making it, so I've probably lost some of what I had in mind. It's especially pointless because I'm reiterating what I've said in different posts (and I doubt anyone will reply). I'm also rushing (I will avoid some details, and explaining certain points that I might take for grated because I'm so used to going over them in my own head).
I think that hedonism is realistically the strongest defense for a pro-MAP/destigmatization of AMSC movement or, alternatively put, is the position that best articulates the MAP grievance in regards to non-violent discrimination against legal adults who feel attracted to minors and the social prejudice against significant age-gap relationships. If I rewired someone in such a way that made it so that they felt viscerally repulsed by the thought of sexual intimacy (or emotional intimacy in the context of sexual attraction) with people they're currently attracted to they would almost certainly not, upon reflection, conclude that a (sexual/'romantic') relationship with those people would be positively valuable (assuming the other person was just as repulsed by them or even found the idea of intimacy with them completely neutral in terms of visceral appeal). If a relationship can have positive value despite causing only disgust/discomfort/emotional distress for both parties (and not being conductive to long-term happiness for whatever complicated indirect reasons either) then being attracted to people you're forbidden from developing relationships with is no great setback or disadvantage since most MAPs (or more precisely, people who have a preference for children, minors or relatively young adults) could almost certainly find some people outside of their preferred age range whom they could develop relationships with.
Happiness is the value that makes the stigma morally offensive (if it's the devaluing of a sexual relationship itself, the extent to which you would want someone to validate the inherent value of such a relationship is the extent to which you are concerned with psychological welfare-emotional validation, and also for something that you would prefer to be a source of happiness for you, although we also naturally want our negative values to be validated by others. If the relationship itself is valuable then it's valuable whether that's recognized by others or not, so the extent to which you value the relationship itself has nothing to do with the validation of others. Wanting validation for the value of something that will or won't exist regardless is 'hedonistic'). Even talking about the inconsistencies in the anti worldview, the false claims about child/minor psychology (e.g. whether or not they can truly consent) or the stereotypes about the legal adults who are interested in them and adult-minor relationships in practice doesn't really address the heart of the matter because it has nothing to do with what makes an adult-minor relationship potentially valuable (as in positively valuable). There are many mistaken beliefs that people have that don't bother us, we might not care if someone insists that the world is flat or that the moon is made of cheese (unless maybe they come off as a condescending know-it-all and/or we want credit for being better informed or smarter in which case that would still circle back to wanting emotional validation for the actual existence of something that would be a source of happiness for you, your perceived intellectual superiority). Insofar as we view a thing as completely intrinsically and instrumentally neutral in value, it wouldn't bother us if other people view that thing as bad because we would be totally indifferent to whether or not it exists. The 'anti' position isn't just a philosophical error, it's a value violation for people who would ideally want relationships with people they are not supposed to be attracted to, under conventional morality. Again, while you could argue that sexual contact or some kind of a socially 'romantic' relationship is inherently good regardless of whether or not it's a source of happiness for people, that is not realistically what MAPs want (to feel attracted to someone is to find the prospect of intimacy with them pleasurable) and if it were, they have the option of that with people they're less interested in (even when it comes to 18-year-olds, and 16/17-year-olds depending on where you live, the societal condemnation of a relationship with someone whom they're interested in wouldn't prevent them from having that relationship so where would the 'harm' be, from a non-hedonistic standpoint? As a general point, the freedom to be with whoever you're attracted to is meaningless if you have such a hard life or for whatever reasons cannot enjoy a relationship with them which is why I think that an anti-ageist revolution when it comes to sex and romance should be tied to a broader movement to raise the quality of people's lives, it's very shallow to me to focus on de-stigmatizing age-gap sex without considering whether or not the beneficiaries of that would still have to deal with an abusive family life or not know where their next meal is coming from or have a place to lay their head or be so crippled by depression, anxiety, grief, health problems, sleep deprivation, various psychological and social issues etc. etc. that that 'freedom' doesn't add to their quality of life).
Even the emotional distress that the stigma can cause doesn't explain why the stigma itself is unreasonable (if you could persuade MAPs who aren't wallowing in shame or guilt that their attraction is wrong it would, in some ways, make it more of a burden, since it's not something they can decide against, but it could also take the sting out of things since you would no longer be violating or invalidating a conscious rational value or preference. A general point about anti-suffering is that it doesn't cover negative value judgments about the things that cause suffering even though people intuitively associate the objects of pain with pain itself. If we assume that happiness requires an object then a pro-happiness position necessarily implies supporting the conditions that enable happiness because of their instrumental value since happiness depends on those things, but it would always be theoretically possible to eliminate the pain that x causes person a by making it so that person a no longer has a negative emotional response to x; or that x no longer causes him to suffer in whatever ways, the ideal wouldn't have to be to eliminate x itself).
Libertarianism is useful in manipulating other people into not interfering with at least some of your goals but I think it's a meaningless position in terms of a value system because it's built on moral relativism (we all decide for ourselves what is good or bad, in terms of the choices we make regarding our property, and in some way respect the values of other people even if you don't 'agree' with them). In one scenario a man is struck by lightening and killed, in another he's killed by another man. In both scenarios the end result is the same, what makes the murder especially tragic? The difference is that the murderer invalidates the value of the man's life in a way that lightening, as an amoral natural occurrence that lacks agency, does not. So libertarianism can help us come to a mutually beneficial agreement with other moral agents but it doesn't cover the value of your life or whatever your goals are rooted in. If the goal of a pro-MAP movement is to merely allow adults to develop consensual relationships with children or minors then libertarianism is enough, just in terms of manipulating that real world outcome, but; again, it doesn't cover the VALUE of such relationships or the immorality in condemning AMSC on principle (which is either because it devalues it as a source of happiness or because it devalues sexual relationships per se).
Consistent libertarians can think that age-gap relationships are vile and immoral and use soft power or non-violent social coercion to discourage them- they can refuse to rent to known pedophiles, to do business with them, to hire them, to shame and degrade them at every opportunity, etc. (they can also make no relationships with adults a condition for allowing their children to reside under their roof). Hedonistic consequentialism can justify discouraging AMSC out of risk aversion but discouraging something because it might cause pain, or enough pain to outweigh possible benefits or the cost of discouraging it, is a fundamentally different position than discouraging it on principle. People will defend the stigma by appealing to child/minor trauma and emotional well-being but that realistically does not cover the conventional intuition that it is bad on principle, not because it causes pain but in itself.
Preference utilitarianism doesn't work for similar reasons (it runs on the same moral relativism that libertarianism; or freedom for the sake of freedom, and democracy for the sake of democracy do). It would require that you consider the preferences of people who are fundamentally opposed to AMSC on principle (how do you give equal consideration to fundamentally different desires? They are incommensurable. You can give equal consideration to the happiness of separate because happiness is happiness). An inconsistently practiced version of preference utilitarianism (which is already internally inconsistent) can be useful in terms of manipulating real world change or as a political move but I don't think it captures the potential value of AMSC.
A vague sex positive position is something I don't think anyone would take to its logical conclusion (if sex is inherently good then it doesn't matter whether or not sexual intimacy is pleasurable, whether or not both parties consent etc. No one really thinks that just the physical contact itself is inherently good).