in all honesty the guidelines are pretty vague, up to interpertation, and as a whole not actually supervised
in regards to sex, it's Technically not allowed. tapas basically draws the line in "sexual nudity" (as opposed to, say, artistic nudity, comedic nudity, or nudity for the sake of plot- for example a character wakes up in a field completely naked and has to figure out where they are)
Here's the mature-specific content guide:
"May contain violence; strong, vulgar, or lewd language; non-graphic sex and nudity; or other material or themes unsuitable for persons under 18." (this is for comics but I believe they're the same for novels?)
from the tapas guidelines, under "content boundaries", under "mature":
not allowed: "...sex or nudity primarily intended to titillate or arouse. Sexually explicit content or pornography is not allowed. Don’t post content that is predominantly sexual in nature, or intended primarily as an aid to sexual gratification."
So basically, sex as a concept is okay, sex scenes not no much, overall- just don't make it sexy (expicit) or the primary focus of your novel.