Thank you for raising this question. I think it's an important question to consider, for any new Stack Exchange site, and in particular where there's overlap as you describe.
That said, I think it really does make sense to have this be a separate site. Here are a few reasons to think so:
- Sexuality is actually something that is afforded relatively little training within the broader training for medical providers, which seems to be the focus of the Health site. Many health providers receive limited sexuality focus during their extensive training, with fewer than half considering themselves "adequately trained to deal with sexual health issues during medical school" (Schindel et al, 2010).
- While certainly many questions about sexuality may be health-related, and/or have health implications, there are also a wide variety of topics (BDSM, non-monogamy, and so forth) that aren't directly related to health, per se.
- Even where there are health implications, many askers of questions about sexuality won't explicitly be thinking of them in those terms, and it won't always be appropriate to guide them in that direction. For example: Some people do experience sexual dysfunction of one form or another because of a health-related cause, and yet many people will have questions about their sexual performance that ultimately boil down to simply not having good information about sexuality -- e.g. not knowing that anxiety can inhibit sexual arousal can, in many folks, create anxiety around their lack of sexual arousal, thus further inhibiting sexual arousal. To direct such a person towards medications, for example, would probably not be nearly as helpful as simply letting them know that it's normal to not experience as much sexual arousal when they're anxious.
There are frameworks around answering questions around sexuality (e.g. the PLISSIT model), as well, which may be more likely to be known about within the world of sex educators than within the world of health care providers. While there's certainly overlap, many (most?) sex educators are not medical professionals, and many (most?) medical professionals are not sex educators. My hope would be that this site would gather some attention from sex educators, to provide solid answers to questions around sexuality. And a sex educator trained in the PLISSIT model will know that there are times to refer out to a health care provider. (That's the "IT" in "PLISSIT" - "Intensive Therapy", in which the sex educator will refer a client out to someone with more specific expertise around a particular topic (including, but not limited to, health care professionals).)