There are lots of bisexual women in San Francisco who have boyfriends and there are venues to enable the fabled threesome. Pleasure Zone is the only one I know of that is having regular, public events; but there are other private parties for couples and single women (no single men are allowed). Other regular coed events to find available bi women are the fetish and swinger scenes.
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Unfortunately, the demand for available, attractive women willing to have casual sex with a couple is very high, and the number of available women is very low. This sometimes leads frustrated couples to invade w4w dating spaces. (This is a no-no, I'll explain why later.) There are informal, private social networks with the kind of woman (sane, attractive, “nice”, etc.) you are looking for but it takes a while to get plugged in. You meet networked people at the above-mentioned events and if they like you they’ll introduce you to their friends. Even if you don’t want to make it your “lifestyle” you are probably going to have to attend events regularly because it takes a lot of hunting to find something rare.
Advertising on w4w bulletin boards is NOT recommended. First of all, it almost never works. A woman who is interested in a threesome can find that in casual encounters (if she hasn’t been snapped up already by one of the above networks). She will get spammed, but she will also get spoiled because the demand is so high. Second of all, it is considered rude. A heterosexual couple is not a “woman” even if the threesome is the female partner’s idea. Even if (perhaps especially if) a woman is bisexual, she is in w4w to get some peace and quiet. Women who are genuinely interested in other women sexually are a minority and easily outnumbered by heterosexual couples. Also, because of the previously mentioned scarcity couples have to do A LOT of advertising. When such posts are allowed/encouraged they quickly become a nuisance. That’s why we have categories. So do not post here. Use “casual encounters” or “misc romance” instead.
If you don’t want to spend a lot of time looking, and you are particular about the other woman’s attractiveness, your best bet is probably to hire a professional. This is especially true if it is your first time and you only want to do it once. There are bisexual prostitutes who specialize in fulfilling such fantasies who will genuinely enjoy having sex with you. Your risk of “catching something” is also lower since the middle- and upper- end escorts in San Francisco are better educated, more aware, and more responsible that your average “sexual adventurer”. However, these women are expensive. If you are cheap, try running an ad in “erotic services” with your photo (a girl might be more willing if she can see you are not skanky) and specify that interested parties must send you their rates.
Here are some helpful websites. You'll have to add the beginning and end to use them because of the asinine robot blockage.
PS: will someone(s) please flag this “best of craigslist”? This question ALWAYS comes up and we need a permanent link. Maybe it will cut down on the clueless couples here.