Is it normal?

Well, I'm posting this just to tell something. Lately, I've been doing something when I fuck my girlfriend. When we do it, suddenly I fantasize and while she's excited with her eyes closed, I open my ass and think about what would be if someone else were watching us. I leave my anus exposed at the mercy of someone who can penetrate me, my erection gets harder. My opened buttocks are available, I've touched my anus and felt that without further stimulation it opens. Well, I wanted to tell this, hope you help