QUESTION: Average length of gay/lesbian relationships

So I was having this discussion with a friend and I told him that Gay/Lesbian relationships statistically do not last as long as het relationships. he didn't believe me and wanted me to back this claim up. It's a known fact in the queer community (and I even remember learning it in a family studies class back in my undergrad days) but I couldn't find any real evidence to back this claim up so . . . I'd like information on the avg. length of long-term relationships for Gays, Lesbian and Straights.