Comforting An Old "friend"?

At one time I used to be one of those goody goods who was like "yeah, I'll never sleep with a person who was dating someone else or married" but sometimes your values will be compromised. My first happened back in 2009, I was laid up post surgery chilling online and a guy I used to know popped up. We used to hook up few years prior and I liked him a lot- he was biracial, long hair, geeky, very sweet, and good in bed..well great in bed. I was surprised to see him after a long time; his mom was sick from cancer, so between working full time and seeing her, we lost touch. We talked for a bit and caught up, he then asked me does my rule about seeing 'taken' men still apply? His mother had passed away a month ago, and he really needed some comfort since his girlfriend didn't live near him, not even in the same state. I faltered for a minute or two..I know it was wrong, but he sounded so lost, heartbroken, sad and vulnerable. I didn't say no or yes but for him to give me a week to think on it while I finished healing from my gallbladder surgery. A week passed and I was feeling better- but the gnawing discomfort wasn't from my scars, but wondering whether or not to go through with it. I hadn't had sex for at least 18 months and I was desperately horny. I just thought to myself, "he needs someone to be there for them- I'll do this once and just once". I was online and he popped on to see if I made a decision, he told me I could say no and he'd understand, but I need some so badly. I went ahead and told him yes. We went back to his place that very night and talked at first over some McD's; I was nervous and deliberately took my time eating wanting to slow time as much as possible and maybe he'd change his mind. When we were done I was nervous and stiff as a board, then he caressed my arm and kissed me. I remembered how soft his lips were and I just gave in, so we got up and he took me by the hand upstairs. The sex was intense but passionless, overshadowed by the fact he kept getting texts from which was probably his girlfriend...the guilt on my part was so thick you could cut it with a knife and serve it. Since it was late and he had work the next day, I opted to crash, and we even slept in the same bed. We fell asleep with him facing me, which I guess was in a gesture of wishful thinking, then he turned away but not before texting some more then finally drifting off. I guess I slept. The next morning, no encore and rightfully so, so I laid in bed with my thoughts as he showered and dressed, and he then took me home. We said our goodbyes and I thanked him with a long, sweet kiss which deflated itself when I caught a glimpse of his girlfriend as a wallpaper on his cell phone. Being the hopeful retard I was, I told him if he ever wanted to "keep things going, just let me know". He nodded and smiled, telling me he'd think about it. A day later online he thanked me and told me he enjoyed himself, but he didn't think it was a good idea to keep anything going for the sake of his blossoming relationship..kinda cliche but the minute he said that, what sounded like a thunderstorm rolled in and it started raining. I understood, although I didn't, he thanked me again and left. That was it, and for the rest of July and most of August I stayed in my room, depressed as ****, feeling terrible thinking I was a homewrecking ****. Well thank goodness for friends who apply to reason, because it really wasn't my fault- he sought me out, he cheated, I didn't because I was single at the time and he wasn't..it was all on him. That and although unusual..very unusual, I was there for someone in their time of need, although a drink and talking things out would've been better than sex, but people grieve in different ways. So I guess I broke my own law for the sake of comforting someone, but I figure it was bound to happen sooner or later. The next one's just a little more complicated than this, if I feel like writing about it...