Consider three ethical claims about sexuality:
Rape is always wrong.
Incest is always wrong.
Bestiality is always wrong.
Take these claims in the strong sense: they hold genuinely always, no matter what the circumstances. Rape is wrong even if the victim is in a coma, is unharmed physically, permission is given by a proxy, and nobody (including the victim) ever finds out. Incest is wrong even between consenting adult relatives raised apart, with no chance of conception. Bestiality is wrong no matter whether the animal is aware of the event, and no matter how low on the evolutionary scale the animal falls or how much the animal wants the act. All these things are wrong even if much rides on them: they are wrong even to save multiple lives, including the life of the victim in case (1).
Now, although it is easy to find ethical views of sexuality that explain why rape, incest and bestiality are almost always wrong, it is hard to find coherent and well-developed views that explain why these are always wrong. But such views do exist. I know of two families of them: there are traditional natural law views and there are views like those of Karol Wojtyla which meld natural law with personalism (my One Body is in this category). However, these two families of views also entail highly controversial further prohibitions on unmarried sexuality, artificial contraception and same-sex sexual activity.
This yields an indirect inference to best explanation argument for the controversial further prohibitions: the best views we have that explain (1)–(3) also entail these further prohibitions.
Of course, one can try to turn the argument around. But I think generally speaking we have better epistemic access to what is forbidden than what is permissible, and so arguing from commonly accepted prohibitions to controversial prohibitions is better than arguing from commonly accepted permissions to controversial permissions.
I am not saying that those who deny the controversial prohibitions need to deny that (1)–(3) are exceptionlessly true to be consistent. I am just saying that they probably aren't going to have a good explanation for why (1)–(3) are exceptionlessly true.