I saw this article musing as to why younger generations seems more comfortable with incest storylines than older ones.
I have my own personal theory and its really that when it comes to sexual content part of the emotional high is about breaking taboos and the more desensitized you are the further you have to push to chase that high.
Younger generations were exposed to so much het/gay/trans porn so early that its kind of become a little dull. It also doesn't help that being gay/trans and watching porn of the same is not nearly as stigmatized as it used to be.
If being merely gay is no longer taboo if its perfectly socially acceptable for you to be watching gay porn then you're just not going to get that breaking taboos bonus especially if you have a supportive community.
So incest is just kind of there still not socially acceptable but under the right conditions can be molded to be morally acceptable.