I think it's the thrill.....the high, rush, excitement that one receives from doing something secretly. Kind of like the rush teenagers experience from doing something secretly and successfully behind their parents' backs. I guess that's why they say that forbidden is fun/interesting. That's one reason, and I've also read that some men (NOT ALL MEN, but typically wealthy) feel like they are deserving/ entitled to finding happiness even if it is achieved through cheating.
There could be several reasons. Maybe the wife is not fulfilling her husband's needs? It could be that the wife is tired from taking care of the kids and has let herself go and there is a lack of effort from both parties in maintaining the spark in the marriage. Maybe things are okay otherwise, but they're not connecting on some level. Or it's simply the newness, ghar ki murghi daal baraabar? I wonder if men realize that the "mistress" could one day become old and boring as well. I wonder if they realize that they have to go through the SAME ENTIRE CHARADE of flirting, giving gifts, comitting, compromishing that they once went through with their WIFE before she had kids and became BORING.
Fact is that there are temptations everywhere. You go through temptations everyday. Either you are strong enough to stay away from them because you understand that it's wrong, hurtful to your spouse, and there is a lot of risk involved, and you fear God. Or you simply are too weak and have no problems taking that risk provided you're never caught. It reminds me of the lyrics from Rhianna's song, "Don’t tell me you’re sorry ’cause you’re not. Baby when I know you’re only sorry you got caught."