I'm not sure if you mean sexuality or sex so I'll just go by what the title says.
A lot of people, including the tapastic community, are at this age in there lives where they're just starting to form their identity and near the final round of cognitive growth. This is usually around the age of 18-25, with younger outliers being the most prominent. That mixed with social media and all these new identities people are speaking out about has fueled the popularity of non hetero-normative sexuality and identity. The "sex" part of sexuality is just one side of the dice. A lot of comics will focus on this genre because they feel a comic about identity/sexuality is a thought-provoking piece of art. Which, it can be, but this sort of genre is becoming so over-saturated that sometimes the "thought-provoking" part is merely replaced by "I'M DIFFERENT YEAH IM DIFFERENT".
Not to confuse sexuality with just sex, which is its own genre that's existed for forever. Sexual identity sells so well because it specifically appeals to an age-group where sexual identity is the most important thing in their lives even if the sexual identity being discussed is not the same as their own. It's about the principal, not the act. Coming out about your identity or supporting someone who is is just super hot and neato today.