Single Girl & The Taken Man

I met this guy at a bar one night we had really good conversation, he was very nice, and was nice looking (which is rare here).  We exchanged phone numbers and after a few conversations I decide to ask the question I should've asked back at the bar "Do you have a girlfriend? Are you involved with someone?"  He hesitated and that said it all.  I told him that he didn't even need to his respond, his lack of response gave me my answer.  He then admitted that he was involved but quickly let me know how attracted he was to me and my whole vibe and I admit I was flattered.  Flattery does get you everywhere however I wasn't buying it at the time.  I let him know that I couldn't be THAT girl and I couldn't get involved with him.  I didn't even want to entertain being his friend.  So that was that... or so I thought.  Clearly NOT.  He continues to call me and plead his case and eventually I start to entertain him.  (I know I'm wrong for that) I start thinking well maybe he is unhappy in his relationship, maybe I am who he needs in his life, maybe ....  Man I think I'm just making excuses and justifications at this point. 
So we go out to dinner and its really cool.  I realize I like this guy but he's taken.  I try to push that out of my head.  We continue having conversations and then I think to myself what am I doing.  This is not me.  I gotta cut this dude off.  And so I do.  I let him know that he needs to holla back when he is SINGLE.  Until then DEUCES. I go about my business.   But this is one persistent guy.  He is relentless and doesn't just quit.  I'm like WOW.  He continues to call and text- begging and pleading and asking for time.  The more he talks the more I start to listen instead of ignoring him.  So we go out again but this time I have my reservations.  I have asked my girls about their advice on dating a TAKEN man.  I even ask some of my male friends and from them I got the same thing- obviously you like him, he knows that, he has nothing to lose, he's going to try until you give in.  And so with that knowledge I start asking him a million and one questions about his relationship, his plans, his intentions with me and when he starts stumbling I start looking at him side eye.  This is exactly what I needed and now I'm really ready to cut it off.  So at the end of the date we say our goodbyes and I call him as I'm driving home and let him know for real this time- THIS IS NOT ME.  I am not that chick who's into dating other people's men.  I'll admit I got caught up because it all sounded good but at the end of the day you are still involved with someone and I can't be apart of that.  Well of course he tried to change my mind for at least a month but I guess he realized I was for real. 
And so what is it with men who are taken that try to get at SINGLE girls like myself? I mean c'mon you are TAKEN. Is it the chase? Is it the challenge? Is it to see if you still got it? Are you that unhappy that home? Or are you just doing it because you can? Whatever the case it's not cool. And I'm not judging anyone because if that's what you do then do you but I will say it's not the single girl's ideal.  I don't like being 2nd and when I get a man I want him to be ALL mine.  And for those single girls like myself who seem to attract these taken men tell them to kick rocks.  You aren't interested.