The beautiful thing about a door is it lets those who want in (in) and those who want out (out). All relationships and marriages are "at will". No one is "stuck" with anyone. We're always where we (choose) to be.
If someone is "unhappy" in their marriage and they (choose) to stay then (they) are (choosing) to be unhappy.
Having said that some people would rather endure unhappiness than to become a "divorce statistic". Others elect to have affairs to fill the void of unhappiness in their marriage. And still others stay in the hopes that things will change overtime. He may feel having any woman is better than having no woman.
A lot of people hate the thought of returning to being single and starting over in the dating pool. The bottom line is he isn't "stuck" he's there because he doesn't like his other "options" any better than what he has. Some people will only leave an unhappy marriage or relationship if they "fall in love" with someone else. Much like leaving one job for another. They dislike being alone especially if all their friends are married or coupled off.