HELP! “I Am Having Great Sex With My Boss And I’m No Longer Interested In Marrying My Fiance”

I had the best sex of my life with my boss – and it’s making me wonder whether I can marry my fiance.
I am 25 and my boss is 35. I got off with him at our office party before Christmas. I knew he liked me – we’d been flirting in the office for weeks. I went outside for a cigarette and he followed me. We started chatting, then kissing, and we were all over one another. We shared a taxi home and he asked if he could stay over at my flat.
We had the most amazing night of sex ever. I have had good sex before but this was something else. I know nothing will come of it, as he’s getting married before I am, but it’s making me think seriously whether I can stay with my fiance.
He is 23, a lovely guy, kind and thoughtful, but he is very small sexually compared to other guys I’ve slept with. He doesn’t have any hang-ups about it so I don’t say anything which might upset him.
When we make love there is no foreplay, and it’s all over within minutes. It is not pleasurable for me and I never think, “Wow, that was fantastic.”
He is pressing on with arranging our wedding but I can’t get up any enthusiasm. I don’t even know if I can stay faithful, though I know he’d be a great husband.
I love my fiance but I also love good sex and he bores me in bed. Should I stay and just deal with being unsatisfied, or drop him because of my craving for better sex?