I have been in the same situation, and to an extent, still am. Two of my very best friend, who are married, and I used to have threesomes pretty regularly. We were all open from the very start and made a point to talk extensively before we ever did because we didn't want to hurt our friendship in any way. After awhile, it seemed like the only time we were getting together anymore was when the wanted to have sex and they both seemed to be getting jealous when I would talk to other guys, almost like I was their "girlfriend". It really started to bother me a bit and ended up cutting it off. I dated someone for quite a few years and they didn't make any kind of sexual advances towards me in that time. After we split up we ended up having another night. I wouldn't let it happen again, just because of the previous situation. What I'm trying to say here is there are so many variables and each of you need to express what you are expecting and what you want out of it. If you are feeling like the only time they want to spend time with you is to have sex, tell them just that. Ask if you can spend an evening doing something else and maybe that will help you from feeling used. Best of luck to you