First cock, missed chance...

I used to live with my girlfriend, we shared it with a gay guy, 1 evening after drinking a couple of bottles of wine, and talking about sex all night, I persuaded him to send me a video of him jerking off, he said yeah sure and I thought that was it, later he left the kitchen, after a while curious, I listened at his bedroom door, and could hear him, I opened the door and there he was on the bed cock in hand, obviously he was like what the fuck, I just walked to him, bent over and started to suck him, but after 30 seconds he stopped me and said he couldn't do because of my girlfriend. I felt terrible for weeks, but after that there was always sexual tension between us. I still want to suck him, and have him fuck me... Whats the worst thing (sexual) that you have ever done but would do again?