The Unfixed Cubefield Why Does My One Unfixed Male Cat Mount My Other Cat?

Why does my one unfixed male cat mount my other cat? - the unfixed cubefield

I have 2 cat, which have not yet been fixed. They grew up with them, but more importantly just started assembling small. I do not understand why he does it. It will not stop until you shut him out of the room and in our room. Can someone tell me why you do that?