Sexual content is allowed! You just have to keep it from being TOO graphic, and you have to put it behind an NSFW-filter!
To quote from the content-policy:
Tapastic ALLOWS mature content featuring nudity, non-graphic sex,
extreme violence, and strong language as long as it is behind our NSFW
filter.
HOWEVER, it also says this:
Tapastic DOES NOT ALLOW pornography or hateful content that targets a
specific person or group of any kind. Any content deemed unsuitable that
is not behind our NSFW will be requested to take action.
So it's perfectly okay to have a comic featuring sex, or that has sexual themes, you just can't get TOO graphic (basically, avoid too many bodily fluids, and don't draw the genitals, and you should be fine), and if you DO have sexual content, the pages with that content has to be put behind the NSFW-filter.
I personally don't have any issue with sex-scenes in comics, as long as they make sense for the story, and aren't just shoehorned in for the sake of drawing boobs, you know? If it's a romance story, sex-scenes make sense! If it's a story about someone exploring or coming to terms with their sexuality/gender identity, sex-scenes make sense! It's all about context. : )