It seems that different cultures have different reactions to mistresses.
Personally, I say do whatever makes you happy... it doesn't matter what my morals are.
Having said that, some things to consider...
Why does a man (assuming a man) need a mistress, if he's married? Is he older? Is he looking for someone younger to play with? If that's the case, how long until he replaces you with a younger model? It's like a car...once they've go so much mileage on one, it's time to trade in.
Does he express love? If so, why is he still married? Why won't he leave her to join his new, true love? And even if he did...how do you know he won't do it again when the thrill is gone again?
At the same time, women for centuries have known men's weaknesses and been able to play them along to get what they want in return for what the man wants. It might seem cheap to some other women (and men) but if they're wired to not let emotions get too in the way, a clever mistress can do well. It's not for too many people though.