So, just an update on my situation. My boyfriend and I had a talk today and I decided that I'm going to let him free to do what he wants, but that I'm going to take a break from the relationship. This is not what he prefers, although he can understand why I would do this.
The reason for the break is because I cannot handle being emotionally and physically intimate with him while he is being emotionally and physically intimate with other women. So I am just going to focus on myself, without having the pressure of being in a relationship, and assess how I feel about him being free to have these other intimate relationships. We both understand that it might eventually break us up for good, if it turns out I can't handle it at all, but I am keeping an open mind that maybe things will turn out in ways that I might not imagine.
I'll try to read up on polyamory, including the Love Without Limits. The book Opening Up seems good too and we might get that too.
Thanks for the two of you who responded. I thought maybe more people would share similar experiences and how they got through them, but I guess those people are looking at other categories in this forum.