You are indeed a pushover. If you're not fine with a poly relationship, you should tell your boyfriend. That being said, there's a pretty good chance that your boyfriend realizes that you're not really comfortable with the idea but still made you agree to it, which doesn't exactly scream of "caring for one's partner". Of course, he might legitimately be oblivious to your discomfort, but unless you're a pretty good actor, it must have shown when you had the conversation. And take some distances with the "friend". They likely knew you had an exclusive relationship and still made a move on your boyfriend. Not cool.

if you are not at all comfortable with the idea, you have every right to tell him no. when done properly, it's an extremely rewarding lifestyle that meets your needs and wants more than monogamy. it does take some getting used to. but it's not for everyone. I have 2 boyfriends, 1 of which is married, and she has a boyfriend. and none of us have ever been happier. if you're otherwise happy with this man, it's worth at least researching to see if it catches your interest. but if he's not good to you otherwise, throw the whole damn man away