Generally it’s because they have taken a strong liking or lust for the man. The reasons for that liking are as varied as there are men and women.
As to why someone would go against the rules of their religion—well, maybe religion isn’t as all-powerful and as wise as most people like to pretend. There is a lot of hypocrisy built into the Catholic faith, in particular. Prohibitions against pre-marital sex and abortion are not supported by most Catholics in the US. The Church and indeed, I would guess that most churches do not have much moral authority within the ranks. It has some authority, but I think that most believers see church teachings as a kind of cafeteria offering—they get to take what they like and leave the rest.
Part of this is probably because of the whole idea that it’s all or nothing. Either you believe everything, or you are a hypocrite. It’s kind of ridiculous, but that’s the way many religions have set it up. All or nothing.
But the reality is that it isn’t all or nothing. We pick and choose what makes sense to us for our lives at the time. It’s a kind of happiness cost/benefit analysis. With a lover who is married, you have many things in the equation. Some include how lonely you might be. How much you like the guy. How close he is to his wife in your perception. How much it would hurt her compared to how much she has hurt him. The impact on his children. Maybe based on your own childhood.
Some people like to act as if it is a single black an white rule that you never mess with another woman’s man. I think the rule makes sense under some conditions, but it is not as universal as people might like to have it. I suspect that people are more likely to follow the rule when they know the other woman. The closer the ties to the other woman, the more likely they will not poach on her territory. But if you don’t know her and she lives in a totally differently world, then what do you owe her? How is it going to hurt you if you horn in on her “territory?”
I think that there is an element of competitiveness to it, as well. Married men are proven commodities. Some woman already desires them. Another woman might come along and may have experienced that all the untaken men are untaken for a reason. So the married man can seem more desirable.
On the man’s side—why do they make a promise they can not keep, or can be tempted to break? Once again, I think that the rules that we have received from milleniums are not necessarily rules that make sense to many people. We’ve been raised to believe the rules are morality without question. But if we were to remake all the rules now according to what really works best in today’s society—I wonder how many people would choose the same set of rules as the best set of rules.
I don’t believe that what people say they believe is what they really believe. I believe that people behave what they truly believe. If someone is stealing another person’s partner, I don’t believe they think it is stealing, nor do they think it is wrong to behave that way. If they did, they wouldn’t do it.
Given the way people actually behave, I’d say that the commonly accepted rules of morality aren’t actually all that accepted. I would not call this hypocrisy, since most of us were never even asked for our opinions when we made up the rules. You might say that no one forces us to make marriage vows, but I would say it is very difficult to go against the rules that have the weight of centuries behind them. It’s easier just to mouth the words and hope for the best, and if you find yourself doing something different, well, you never had any say in it in the first place.
Life as she is lived is different from life as she is writ. People do what they think is best for them personally, not what the rules say.