Why do people care about other people's sex life?

I just want to ask. <br>I was in a room full of men when a girl tried to leave with me (I am a man by the way, this is important for context, <br>I am mildly facecucked, however, I am extremely socially, financially, and physically gifted). <br>However, this girl had a boyfriend who she was clearly cheating on. <br>One guy at the table said "bro this girl has a boyfriend leave her alone" because he liked the girl. <br>Another guy in the room backed me up and said, "not my business, I have a gf I'm good."<br>I ended up taking the girl to my bed, and things happened. <br>I just wanna ask in general: <br>Why does it matter to anyone if a person is having sex or not?