I need relationship advice help??!?

OK so I’ve been with my boyfriend for four years and we’re very happy. We’ve even talked about marriage and starting a family when we get older because right now we’re only 18. And recently we’ve been hanging out with my uncle who is dating somebody who is 20 and my uncle is thirty-four I believe, and the other night my boyfriend and I were hanging out with him and his girlfriend and he was talking about how they’re in an open relationship. He asked if we think it’s weird because you know he’s a grown man and he’s in an open relationship and I said I just can’t imagine being in one because I’m already insecure and I don’t like to share my boyfriend. Like I’m the type of person that might offend people by saying I think you can only be in love with one person. But when my uncle said some thing about being in an open relationship my boyfriend had a weird look on his face so I asked him about it the next day. He said that he’s thought about it and he’s considered it and he wants to do it but he knows that I won’t do it so he didn’t bring it up. And he obviously knows how I feel about it and he said that the only reason he didn’t bring it up is because he knew it would upset me and he didn’t wanna start problems. But of course me being me sensitive insecure I text him while he’s at work and ask him you know why do you wanna be in a open relationship what you wanna do is it gonna be dating or just sex what am I missing that you still want? he said it would just be sex and it’s hypothetical and he didn’t bring it up because he knew it wasn’t gonna happen I just wanna know if I stay with him and should trust him to be loyal to me? So what I’m asking is am i overreacting? should I just drop the situation and stop thinking about it? because it hurts me so much to think about it that I cry.I just don’t wanna lose him. I want honest opinions please. Also I might be pregnant but I’m not sure. I’m buying tests tomorrow so I know for sure.

So please read through this and drop some comments on what you think about the situation or your advice or what you would do in the situation just so that I can think about what I wanna do.

thank you