In love with someone elses girl

So lately I've been hanging out with this couple I met at my workplace, the girl is very hot this is a young couple, and I fancy his girlfriend like crazy...

and the other day they were going out and the boyfriend was already in town waiting for her and she messaged me and asked me to drop her off. so being the nice guy I am I picked her up she looked so beautiful, hot. anyway she told me to park up in a back alley on the way, and she pulled my pants off and you can guess what happened next... anyway I enjoyed it and dropped her off..

now it feels weird when ever I see the guy as I love her so much, but I don't want to be with her just have a sexual relationship that is all... is this wrong?