IMHO

"I'm in love. She was once married. She had an affair. She has been very honest with me with everything, including the details of how the marriage failed. Now I've been dating her for about 8 months. Something has happen that I wasn't expecting though. She is still in contact with the loser she had the affair with. But she seems almost willing to let our relationship crumble if I don't just accept she is going to continue being friends with him. What the hell is going on here, am I being stupid for suggesting she let him just move on or am I being an idiot about to get burned as usual?"

Would you trust the girl if you were in his shoes? It has all the warning signs of either and existing affair or an affair that can be ignited at any time. Simple example. If your dad has had an affair, will there be any effect on you in the future? Issues in trusting your partner?

People who have affair just didn't have a conscience about their actions, no respect for relationship boundaries, lack of values and most of all irresponsible. It's hard enough to deal with the random girls at bars, his workplace, his ex's, silly new relationship insecurities, etc. Why bring another negative into the equation? Let's face it. Back to the problem stated, nobody knows if she will actually do something but the fact that he believe she is CAPABLE of it is the real problem. Is she the kind of person you want on your team or not? Figure that out. Life is too short to have to watch your back amongst friends.

A married man that has an affair is usually doing so out of his own selfishness and personal needs. He will TELL YOU ANYTHING you want hear to keep you interested and will string you along until he is done with you or there is an option of being caught.  Talk is cheap. He can say he wants to keep your marriage intact but if he is not willing to take whatever action is needed then his real desires here are pretty plain. Then I'd be so sorry of this misfortune.

IMHO, people who do affairs, does not understand love to begin with. By cheating on your partner and having an affair, is simply showing that they love themselves more than they loves other. They are chasing the dream of self fulfillment and the so called 'new age' religious thought process of making ourselves happy and putting ourselves first and foremost. Love is not self-serving and all affairs are. Most people are misinterpreting the definition of love, they want to be glorified and not loved.
 
Some can't just have one partner in their life so they cheat. Some men may well love their wife, but, instead of communication to her if he's unhappy he may have an affair (it isn't always love) with another woman but to fill his sexual needs. Some men in the 40 year old bracket can have what they call 'Andropause' (similar to women's menopause) and feel they have a need to prove they are attractive to the opposite sex and also to do some of the things they missed out on in life. Although this is still not an excuse for cheating this is a dangerous time in some men's lives.