I Got Off With My Roommates Gf

In the summer between high school and college I had a roommate who had a hot gf and she'd stay over and I'd hear them fuck. It drove me crazy. They fucked one morning and I thought they both left. So I was jerking off out on the couch because I was horny and thought I was alone.
She was still there showering and walked out on me, going to town on myself, and I am pretty "equipped:"
I first we were both shocked, then embarrassed... I started to apologize. She kept saying it's ok, it's your home. It was so awkward, and she was just as embarrassed and awkward as me. Neither knew how to get out of this embarrassing situation. But I think because it was awkward, we were both apologizing, and it was going in cycles she finally said, "It's your home. Keep going." And she dropped her towel exposing herself and sat in the armchair across from me spreading her legs so i could see and blushing like crazy. I tried to stay, and we shouldn't do this, but all she said was, "Ssshh. We are not doing anything together, you stay there and I will stay here." She was super hot, and to she blushing and exposing her pussy to me so i went back to work on my dick. We both just masterbated and came. As soon as I shot my load, she covered herself again and mumbled in an ashamed way with no eye contact, "Please don't tell him."
And she got up and ran and got dressed and acted normal like nothing happened. Nothing changed between us at all.
Three more times that summer, it happened again. I heard the front door close, and the shower run, and I dashed for the couch. She'd find me beating off. And would causally sit across from me and spread. No words, except mumbles and moans were exchanged. I stayed on the couch, and she stayed in her chair and she moan about how big my cock looked. We both moaned and mumbled when were were about to cum. We'd cum. She'd dress and pretend nothing happened.
I wanted to fuck her soooo bad but I needed to keep my roommate so that's as far as it went.