It wasn't the first time I had sex. It was the first time I time traveled (as far as I could remember anyway).
I had decided to pick Xi Tan's wedding because that was the first time the idea had occurred to me. I guess her body was irresistible in those days, but when time is malleable, bodies come and go, but it is the soul that's eternal. She was getting married to someone else. It was the sight of her naked back in her unzipped wedding dress that made you truly appreciate the artwork that is humanity.
I'll have to add the details later, but suffice to say I managed to find her away from the other wedding guests, and that's when the magic happened. It actually wasn't the first time we had sex in my timeline, but it was the first time we had sex in hers.
Maybe that's why she was so attracted to me when I first met her before I could time travel. Or maybe it was just that our souls had already been so intertwined with one another throughout the mess of history that was created after time travel became common.