Good friends vs. romantic emotions

I have a conundrum for you.

It was my second semester at college. The first day of one of my classes, I met this guy, Zach. It was pretty obvious from the start that he had an enormous crush on me. But at the time, I had a boyfriend, Teddy. So obviously nothing magical was going to happen between us. A month or so later, Zach came with me to a bar. There he met my best friend, Jackie. The two hit it off immediately and were making out halfway into the night. A month later, they were inseparable. After they started dating, Teddy and I broke up. Zach found out and began asking me to hang out since we lived in the same dorm.

Once, Zach tried kissing me, I stopped him because, duh, he was dating my best friend. He told me he always liked me and Jackie was just a back-up. He broke up with Jackie and asked me out on a date. I wanted to say yes, but Jackie was heart-broken. Never again did I talk to Zach and Jackie and I are still best friends. Yet, I've always wondered what could have been...

So how do you decide when dating a friends x-boyfriend is acceptable or not??