At first glance, this site appears to be all about couples, and few of the regular posters are openly bi or gay... but take a look at the numbers of photo adds:
5208 Men seeking Women
3144 Men seeking Couples
2227 Men seeking Men
1785 Couples seeking Couples
1153 TV/TS/CD and Fetish
857 Couples seeking Women
597 Dogging
569 Couples seeking Men
332 Women seeking Women
156 Women seeking Men
147 Men seeking Groups
86 Women seeking Couples
52 Holidays
41 Clubs and Parties
37 Couples seeking Groups
11 Women seeking Groups
Breaking things down a bit:
1785 Couples seeking Couples
857 Couples seeking Women
569 Couples seeking Men
156 Women seeking Men
11 Women seeking Groups
= A total of 3368 ads by couples or straight women (many of which specify respondents need to be bi)
2227 Men seeking Men
1153 TV/TS/CD and Fetish (I'm generalising here, but these are mostly m4m)
332 Women seeking Women
147 Men seeking Groups (again mostly groups of men)
= A total of 3859 ads for same-sex fun
(of course the 5208 Men seeking Women and 3144 Men seeking Couples outnumber both categories, but we all know they don't have a great deal of success).
It looks like the gay/bi community is actually in the majority here!
Maybe I'm perpetuating a stereotype of promiscious gay men (what do I know? I'm only a promiscuous bi man!) I'm guessing that the Men seeking Men section might well be accounting for the vast majority of actual meetings arranged through SH?
(oh and I know I've made some pretty sweeping generalisations and assumptions in the above, but i'd have ended up with some horribly tortuous sentences if I was trying to be precise and account for all possibilities, so I hope you'll forgive me for that)