She is not his wife, she is not mother to his children, she is not his parents' daughter-in-law.
Don’t let the temptation to think that there is more to this than a no strings attached relationship cloud your judgement.
One of the first things you need to do when dating a married woman is to dress to impress.
When she believes her husband has started taking her for granted, the fire in the relationship begins to dwindle down to just an ember so she is not looking for just another ho hum guy.
So the married woman/single man coupling is the more rare circumstance. Why do some single men have affairs with married women? Unlike most women, single men often love having access to someone who is passionate, sexual and really wants nothing more from them. The goal of dating for married people is far different from dating among the singles. This is the most apparent reason why they willingly throw themselves out there.
On a purely physical level, most men are going to enjoy their affairs with married women. You’re unlocking a woman full of sexual desire who is not getting her needs met at home, so of course the one night (or many nights) of sex is going to be unbelievably great. You don’t need to meet the parents and you won’t miss a single bowling night out with your buddies. But for married people who are already committed, they are just out to seek solutions to their unmet emotional and/or sexual needs.