Because married men get laid more. They are also healthier and wealthier.
([url=http://www.webmd.com/sex-relationships/news/20100224/single-men-have-higher-risk-of-stroke]Single Men Have Higher Risk of Stroke[/url])
When you're single, you're also going to need to do the whole mating dance EVERY SINGLE TIME you want to get laid. That means taking a girl out, romancing her, dining her... it takes time, and money. And there's also no guaranteed pay off. So you're potentially wasting time and money. Yes, you could eventually settle into a relationship. But generally relationships led to monogamy and marriage. So if you choose to break off the monogamy, then you're right back at square one.
Eventually that good looking male is going to get older and lose his looks. When the guy is 50, 60, 70, are the hot women REALLY going to be sniffing up his tree? Maybe if he's incredibly rich, but only then if he's willing to share some (ie, marriage.) Guys who are married, however (HAPPILY married being the important factor) don't usually need to woo their wife with superficial charm. They have years of love and companionship (and if it's a good marriage, happy memories of acrobatic sex) to keep the fires burning.
Yes, single guys potentially get more variety... But married guy gets quantity. They also get stable finances (two incomes, generally), and people to look after them (children, which women USUALLY won't have with a guy who isn't digging the marriage thing.)
Also, evolutionary biology and psychology are complete bs. Just throwing that out there.