Have you tried the "google groups" or "groups google" website.
Do a google search to find it if you don't already have it bookmarked.
Type in "human sexuality"
Write down the names of the newsgroups. Subscribe to the ones that are
related to human sexuality. If it becomes obvious that the people in the
newsgroup are not interested in an honest discussion--unsubscribe.

Per pauldepstein@att.net:

Seems like the nature of newsgroups mitigates against finding anything that's
not spammed - no moderation, no control.

If I were looking, I'd try to find a mailing list - most of which are moderated.
Google hosts a lot of lists and my guess is that with a little searching you'll
come up with something.

soc.sexuality.general

If you're considering a mailing list, you can pick one at
http://lists.topica.com/search/?query=sexuality&search_type=cat

I've had bad experiences with moderated and non-moderated newsgroups,
but at least the moderated ones are more mature. The internet is a hell
hole and only the communists care. We're all doomed.