Because there is nothing that binds two men together besides sex, and even that fails to hold gay men together because gay men can get sex so easily - because men are horny dogs.
A straight man remains with woman because women have babies, and a straight man derives a sense of power from feeding and protecting the women and children under his roof. So the man sticks around because he understands that his influence is necessary for his sons to grow up into alpha males and for his daughters to not grow up to become sluts. Gay men, even when in love, have nothing to bind them together, and the temptation to get sex anywhere and any time you want is too great to let go.
But gay men could stay together even without kids, but the problem here are two. The first is the issue of sex. Gay men can get sex much more easily than straight men, so the temptation to strain is great. A straight man, unless he is famous or extremely handsome, cannot get sex anytime he wants, so there is an added incentive to stay in a relationship. The second issue is masculinity. Straight men are usually masculine, so they are attracted to the femininity of women and the relationship works. As they say, opposites attract. Gay men, conversely, are almost invariably effeminate, and they end up in relationships with other gay men also effeminate. Equals do not attract. There is little incentive for two effeminates to stay together because people are attracted romantically to people who posses the characteristics of the opposite sex they identify with in a psychological way. Lesbians also complain that there aren't many feminine lesbians around.
