My roommate doesn't want me banging her friend.?

I live with a female roommate and we get along really well, 98% of the time. The only thing we ever argue about is her best friend. When I was helping her move in I overheard her telling her friends that I was "off limits." Which would have been ok had it not been for her best friend who happens to be a knock out. All her friends like me and I go out with them all the time, which gets tricky after a few Jack and cokes. Maybe two weeks ago I hooked up with her best friend after the bar. Since then we have hooked up maybe 6 times. I always go over there so the apartment doesn't get all awkward. I thought everything was fine until we got busted. My roommate freaked out. After talking to her about it I feel conflicted. Half of me (guess which half) wants to continue seeing her friend. The other half feels really bad for having to sneak around. I don't think my roommate wants anything from me other than friendship (she just got out of a 3 year relationship). But she obviously doesn't want me doing her friend. Do I owe it to her to stop with her friend. I've gone out of my way to keep it low profile but now she's all passive aggressive. This sucks. What would you do, and why would you do it?