Am I overreacting My bf and sister are having a baby

My boyfriend gave me hypothetical scenario a couple months back and it’s been causing issues ever since…

About 10 days before the 4th of July, my boyfriend brought up a conversation I have not been able to forget. He asked wouldn’t it be cool if a kid grew up in a family thinking their dad is one guy but really the dad is the uncle. I said WHAT?! He then used my family as an example, he said like if your sister got pregnant every would think it’s [blank] (my sisters current lover) but really the kid is…. And I said yours ? Is there something you need to tell me ?

Obviously, I asked my sister who I haven’t heard a word from since. My family is distant and now my boyfriend pretends he never brought this up.

However, he is gaslighting me for sure because I remember how upset I got at this hypothetical scenario. It ruined our night and ultimately I think our relationship.

Does anyone have any sound advice how to handle this situation?