Is it normal to like to watch your girl with others?

Even when I'm having vanilla sex with a girlfriend, I find myself fantasizing about seeing her with another man. I've made this a reality several times. The experiences ranged from "okay" to mindblowingly erotic. And usually the girl enjoyed it far more than typical one-on-one sex. (Often women are surprised to find how much they enjoy two men.)

Of course there's a taboo here, and not everyone is comfortable breaking it.

I wonder how many men enjoy seeing their woman with another man or fantasize about it, and how many women enjoy being shared or watched and eventually having two men either at once or one after the other.

Is it normal?