You asked a good question. I don't understand why men expect beauty at all cost, but I recently read two books, "For Women Only" and "For Men Only" that explains a bit about it. If you want to hear more about them, I reviewed them at: URL: http://www.bellaonline.com/articles/art47166.asp
However, even though I don't think it's fair, it's a fact of life and it will take a long time to change these attitudes. I'm also very impressed with the Campaign for Real Beauty conducted by Dove. I've been reading material from their international study on this and it seems if we want to change people's attitude (men/husbands included) we need to start with our own attitudes (women/girls/mothers). I believe it is as much a self-esteem issue. I intend to write on the subject, but want to do more research.
However, back to your question about men turning to other women when their wives aren't as beautiful as they want, it does happen unfortunately. But, I personally think that a man who is prone to cheat, will cheat for any reason. In fact, I have known men who have had beautiful wives who have cheated. It's not just because of how a wife looks, sometimes that is just an excuse. I honestly believe there are some men who would cheat and blame it on the size of his wife's feet if he thought that was a good excuse.
So, what I'm saying is yes, it does happen. But it's not all because of a woman's looks. It's the man's moral fiber.