Is it normal to masturbate to your girlfriend fucking other guys?

My girlfriends never cheated nor given me reason to believe she has but for some reason right as I'm getting off I always climax to the thought of tons of guys fucking her in as many degrading positions possible. Usually climaxing on her face or inside her, giving her child an anonymous father.

I love my girlfriend and get angry at the slightest thought of some guy so much as looking at her, however somehow it turns me on in the heat of the moment to think of her as a degraded slut.