LW#5: Your boyfriend sounds nice, and heaven knows we've all done things we regret. There are times when paying for it can be so tempting, if you're really lonely (lord knows I've been there, I'm 33 and still a virgin). I've spoken with other guys who say that they prefer sex with prostitutes, which I find a little creepy, but they sound 180 degrees removed from the kind of guy that you seem to have found.
I dated a bisexual woman for awhile, and we kind of had a similar conversation. When I asked if I could be "involved", she explained to me that she was trying to be open and honest about wanting to explore a part of her sexuality, but was not happy that I took that as an opportunity to make it about me. And she was right: she was saying that she wanted to have sex with a woman, not have a three-way, and it was not cool at all for me to immediately go there.
I don't know all the particulars, LW#2, but maybe your boyfriend sounds a little greedy?