Very very rare by themselves. I have been around escorts for a very very long time, many of them bisexuals, promoting their services trying unsuccessfully to get women as clients. For my part, I have seen women exclusevly with their bf, husband or lover and I am a true bisexual. When I see couples, the women are really into it tho which is really nice. Couples seems more and more frequent, I don't know why but I had a significant increase of requests coming from couples the last 2 years, both young and older couples.
I had a few requests coming from single women here and there, but I refused them all because the emails were too intense, too much personnal details, too much of "your pics remind me of my ex" etc ot the women were too young for me.
I have to admit that women are generally not better than men when it comes to reading a website/ads and that some sound really od when contacting me by phone (generally calling to meet them with a bf).