Imagining others having sex

My new boyfriend (together two months) and I were watching a movie with multiple sex scenes last night. He asked if I was up for playing a game. He suggested that for every couple we saw in public, we would tell each-other whether we could picture that couple having sex or not. Like imagining how it would be for that couple having sex. I said absolutely not. I found that to be very strange. I should have asked him why he would think of that but was caught off guard and didn’t address it.

We are active sexually and enjoy each-other often.

Does anyone else think this is weird?