Commited and Flirting?

Friday night I was out with my roommates hanging out and having a good time. Jamie and I had been at her work holiday party earlier in the evening and had planned to meet up with a few people from the party at a bar in Denver later. We were the first to arrive at the bar and sat down to get drinks while we waited for others to show up. A couple of guys we knew came over and started talking to us. Then Jamie's coworker and his buddy showed up. The buddy, Joe, started talking to me and we hit it off pretty quickly. I was thinking, hmmm, I think this guy may be into me. Then, 40 minutes or so into the conversation, Joe says something that most flattered single women don't want to hear "my fiance". What? Fiance? Why are you standing here buying me drinks and engaging me in conversation if you have someone who loves you at home?

Well, boys and girls can be friends right? So I figured, as long as the company was good, there's no harm being done.

We decided to go to another bar, where, amazingly, we ran into more people we knew. I moved from conversation to conversation chatting with people I knew and people I didn't (I become a bit of a social butterfly when I drink) and I somehow always ended up talking to Joe. About an hour before we left, most of our other acquantainces had gone and Piper, Joe and I were left sitting at a table talking to random people who would stop by. Joe inquired a little bit about my personal life. I didn't give him much information other than that I'd broken up with my boyfriend a month ago. I was a bit taken aback when he asked me what the 'asshole ex' had done. I said he wasn't an asshole (I'm of a different opinion now). Joe looked surprised and told me he had just assumed because anyone who could break up with a beautiful funny girl like me must be an asshole. Maybe I'm a skeptic or tiny bit cynical, but that sounds like an awful nice compliment to give a girl you hardly know if you don't have an alterior motive in mind.

When Piper and I finally decided to call it a night Joe walked out with us. He said he'd had a great time and hoped we could meet up again sometime. Of course, being engaged and all, he couldn't ask for my number, so he said something like "I'll have Jason call Jamie and set something up sometime".

So, to get to the point, why was a cute, funny commited guy interested in me? Is there something I am missing? I wouldn't mind being friends with Joe, but I also don't want to be a strain on his relationship. I wouldn't want to be the girl who broke up another girl's relationship. Especially since something like that (sort of) happened to me recently. Do guys and girls think so differently when it comes to being in a relationship and making new friends with opposite members of the sex? When is it okay and when is it not?