Is it wrong to date a friend's ex?

At present, I am sort-of seeing the ex-girlfriend of a good friend of mine. The girl in question has been my friend for ages, longer in fact than the other guy involved in this story. We have been seeing each other for a few weeks, and tried to keep it on the down-low, especially from her ex. Her ex, my friend, is currently in college, and it is not often that I get to see him. I had hoped to keep this from him, mostly to spare his feelings, but also because it is not a serious relationship.

He found out, however, as I should have seen coming. I realize now that not telling him the truth, especially after he asked me about it, was a low move on my part. I am willing to admit to that mistake. But honestly, “Guy Code” aside, is there any reason to feel guilty? I am upset that I have marred a friendship, but can I really blame myself for the way I feel? I accept that lying about it was a boneheaded move, and I regret it deepy, but I do no think my seeing this girl is something that needed approval, least of all from her ex-boyfriend. I hope that my friendship can be salvaged, and hope that we can move past my mistakes, but I do not feel sorry for the way I feel. I denno, I’m rambling lol. What experience do you guys have on this subject, and what are your feelings?