Is it normal to like cheating women?

Cuckold fetish, possibly. If I had a partner that cheated, I'd hire someone to get it on video, for research purposes.

If this applied to a girlfriend of yours, then how would you feel knowing her vagina and mouth were recently penetrated by a penis?

I can't think of anything more repulsive, personally. But when I was 18 I had an affair with a married woman. Mostly I felt bad for the kids and husband, but I really blamed the wife more than me.
She was the perfect example of the virgin at marriage, who got curious and fucked up the lives of her 4 kids, her husband and herself, just for a bit of strange. What a waste.

I think you like the idea of an open marriage. Or, you just have a thing for someone else fucking your woman.