<human>: I just need to hear other people's thoughts on this. I have been with Alex for about 9 months, and we are in love. 

This happened a week ago and I still have not been able to get over it. He basically sat me down and asked me to have a threesome with him and one of his acquaintances, Grace. I later learned that he's slept with her in a threesome before. He and his old girlfriend (Kelly) found Grace online through some dating app. They aren't really friends or anything, but I learned that they would periodically invite Grace into their bedroom for the experience. 

He said it was completely my choice and that we obviously did not have to if I wasn't comfortable with it. There's no pressure on me at all. I told him I would think about it.

Here's the thing: I am *really* into monogamy. When I'm with somebody, I don't even *look* at other guys, and I could never IMAGINE wanting to have sex with somebody else. I don't judge others for wanting threesomes, it's their bodies to do with whatever they want. But I, personally, am kind of repulsed by the idea of sharing another woman with my boyfriend. 

Honestly, the idea that he would even consider sleeping with somebody else in addition to me is really turning me off, to the point where I don't want to be with him anymore. I just feel like we are incredibly incompatible now.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Boyfriend wants a threesome with an acquaintance; I am so completely against the concept fundamentally that I am thinking of ending it with him because we are incompatible.