What is the attraction to the "FORBIDDEN fruit" ?

Hi IN members

I was at a cafeteria having brunch with a friend and we we couldn't help overhearing a conversation from the table next to us. From what we could hear, it appeared that one of the dinners was pursuing someone who for aguements sake was "married". Nothing wrong with that, but the person being pursued is unavailable (married/committed to another) and have made it clear to them of their situation, but this person was determined to win them over and have them. This man was determined to win this woman from her husband/partner.

I once went on a date with a guy some years back. We met again another couple of times. I thought we got on so well that I suggested meeting again and at this point he bluntly pointed out that he was not interested in a relationship with me etc. wow! I was shocked, but ok with that. I replied that I was in a relationship and wasn't looking for one with him either. Strangely, something changed. All of a sudden he was interested, he wanted to take me out, spend time with me etc. Of course I declined and moved on as I knew he was a game player and was only interested because I said there was another and I was not interested in playing games.

My question here are:
* why do some people actively seek out people who are committed to others for example men who pursue married women and women who pursue married men.
*What are the attractions to married/committed spouses that makes them so attractive to single men and women?
*Why the change of heart after knowing someone is not available?
*What changed?

Your views are greatly appreciated. Let's discus :)