<human>: Can you provide a brief rundown of the text in within 3 sentences?

My boyfriend and I have been dating for three months - short time, I know. But he is the best thing that's happened for me. We met because we have a class together at our college, turns out we went to the same high school and saw each other every day without even knowing. Small world. :) He is 19 and I am 18. 

Our sex life is wonderful, at first I thought we wouldn't be compatible in bed because we both enjoy domination from our partners. However we made it work and just switch roles at random times so it's never dull. I've gotten him to get more aggressive though so we're making progress and training each other to what we like. 

Back to the point, I've always considered myself straight, but the past year I've gotten a little bicurious. I like to keep an open mind and I guess it caught up with me. :) I don't think I could ever date a girl or love her, but I've fooled around with a girl a few times and it was great. I would definitely like to explore more. I finally told my boyfriend this a few weeks ago and asked him what he thought about a threesome. (he has never had a threesome, I've only been in an mmf threesome so I'd like to experiment and try ffm). We ended up fighting because he said that a threesome would ruin our relationship, he wasn't going to cheat on me right in front of my face, among other things. I was upset that he shut me down and that we fought but I dropped the subject. Then last night we went on a date and on the drive back he asked me who I'd like to have a threesome with. I was surprised because now he's suddenly changed his mind. 

I'm scared he could be right though, and having a threesome could ruin our relationship. So my question is, how could we go about this without ruining our relationship (or putting strain on it)? Did he change his mind just to make me happy, or does he really want this too? Are there any specific guidelines we should follow? I just need advice.
<bot>: Asked for a threesome, boyfriend immediately said no. He changed his mind now and said yes. How do we have a threesome without causing strain on our relationship?