You are about to learn some painful lessons

signed The Voice of Experience

If you are uncomfortable being number two then you should not settle for it. I thought my lover was going to leave his wife for me and I ended up with so much pain and anguish. Men stay with their wives because they are content with their life at home. Trust me if he was truly unhappy he would have left her already. I have knew a man that walked away from a wife and 5 kids and didn't bat an eye to pursue his "true " love. Don't let him hand you a load of bull about his "obligations" etc. Men are great liars and they will say anything to get what they want. They will whine and bitch about their spouse yet they go home to her every night and foot the household bills and take her on vacation. You need to ask yourself if you are getting what you want out of this relationship or are you settling for table scraps.

we cannot choose who we fall in love with. However, just because we are in love with one person, it does not mean that we cannot love others equally much or more. You could walk away from this one, and love another, someone more available.
Or, like pollygarter you could continue, knowing that he will never leave his wife, knowing that the fallout could be immense if she found out. If you do that, then make peace with your conscience. If you can't then listen to, and move on.

We each decide our own destiny.

I am an unfaithful wife.
Yet I do not wish to be without my husband.
Who or what decided we stay sexually faithful to one person?

I fall into and then out of people's lives....intimately...sometimes I fall in love...and sometimes, wiser now, I remove myself from the inevitable falling in love as it breaks my heart and I've been broken enough already.

My soul needs feeding sometimes. x

Going back to your original question - there seems to be lots of evidence of infidelity by both sexes. I suspect though men are more easily tempted to no-strings 'extras' if offered on a plate and they think they won't be found out. I may of course be totally unfair!
By nature I am a serial monogamist, and I guess the reason I'm a mistress is that I met someone who 'feeds my soul' as well as the rest and I think his wife actually has a pretty good deal out of her marriage. Personally I'd prefer her to know I was in his life, so she could make informed choice on her future, but it is for him to tell her, not me.