Personally, I don't mind talking about/reading about sexual subjects on here at all and I also think that it's far better that young people come on here and read the fairly measured comments most of us make, rather than going to other sites, where the content may be far more unsuitable for their age group.
However, whilst I agree that this forum has become sooooo much more than just a bag forum, I also feel that an adults only subforum (private or not) is probably too specialised an option to be considered an urgent priority; especially if it would create problems and/or a lot of extra work for the admins and mods.
If this was a health and fitness, or relationships, forum, I think it would make perfect sense to have a private subforum of that type, as to not have one and to disallow explicit conversations about sexual matters, might make it seem rather limited; but as others have said, it's not, it's a bag forum.
TBH, I'm a bit confused as to why, mostly female (I assume?) members of a fashion forum (of any type) would feel the need to talk about sex in overly graphic terms and/or to use explicit words, anyway?