There's a whole of reasons why you can't compare their situation to yours:
* Social Pressure: The societal pressure to get married to a member of the opposite sex in our culture is tremendous. Grandparents ask, "So, when are you getting married," or, "When are you giving us grandchildren?" So, to come out as a gay person, you've already bucked the system by coming out as gay. This throws a wrench in social convention.
* Coming Out: I'm guessing you didn't start dating guys at the same age your friends started dating members of the opposite sex. So, they've had more time to play the field, meet that special someone, and settle down. You probably also had less of a dating pool to choose from than they did. In other words, they had a big head start.
* Marriage: Of course, as a gay man, it's not easy to get married -- or have (or adopt) children -- in most parts of the country as it is for a straight person.
* Priorities: Many gay men aren't interested in marriage. Many gay men aren't interested in children. Or, at least, get to it at a later age than most straight people, though this is changing.
But, let's face it, who knows what their relationships are going to look like in 2, 5, 10 years. They could be divorced and remarried in that time. Marriage is a nice ideal, but marriage isn't perfect. There's no guarantee they're going to be happy.
So, I wouldn't beat myself up over the comparison. Yes, I've been where you're at. It is a bit odd to have straight, but even some gay, friends in a different place in life than you are. It is an adjustment. But, you can't live your life compared to the way they're living their lives. You're on a different path. That doesn't mean you're not going to end up in a rewarding place on your own. So, don't sweat it. Accept that "different" doesn't mean bad or less than what your friends are going through. It can be rich in its own right.