Is it normal to be attracted to my friend's girlfriend?
So I have this friend I've known for about 5 years. We aren't really that close, we just sometimes party in same groups. He has always been really nice to me. During these 5 years he has had a relationship with 3 different girls and as soon as I have seen them together I've instantly started to get attracted to the girl (mostly in the sexual way). One of those girls I knew before they started to date and I didn't have any feelings for her, until after I heard that they are together.
Nothing have I done to disturb their relationship though.
Sometimes only jacked off for their pictures and felt a bit guilty for that afterwards.
I have no idea why it has only happened with his girlfriends but not any of my other friends'. Could be just coincidence? I really don't have anything against this my friend.