As for limits, my boyfriend and I are very open when it comes to receiving the others' sexual ideas. It's all sort of weird, actually, how we talk about our sex life in such a nonchalant manner. We have long and constructive discussions about any fantasies one of us might have. Although I have a little bit of trouble being this open, he never seems too embarrassed or scared to talk about our sex life with me, and I take comfort in that honesty.
Here is something about threesomes that my boyfriend and I discussed. When it comes to a guy with two girls, you often expect the girls to interact with each other sexually. I think this is one of the biggest turn ons about a threesome. Well the reason my boyfriend never tells me he wants a threesome, is because he feels he would have to indulge a similar scenario for me if I wanted him to: Me, him and another guy, with the expectation that he and the other guy would interact sexually.
Just like any part of a relationship, there is a huge equality thing. When he asks you to go beyond your limits, question him if he would do the same for you in a situation that he is sensitive about.