With this type of thing heavily female is not a problem -- if you get the women, the men will come. It's why OKCupid's design is pink.
The ratio of active, real men:women for online dating is 10:1 or sometimes 100:1. So targeting heavily female niches is fine. The men will find it.
Yeah, that's actually a really good point. I can't think of any good examples though where this would work. Like if you go to Borders and look at their magazine racks, there are tons of specific niches that cater to mostly, if not all, female (there are like 8 different magazines on knitting), but I'm having trouble seeing how you could apply it in the dating realm.
My friend I came up with the idea of a dating site based on what your fashion sense is - t-shirts/casual personalities being more compatible.