Matan1 – there was a 2003 study done in the journal AIDS by Dr. Maria Xiridou about this. Study found that average length of relationship of gay males is 1.5 years. Additionally, the people in these relationships averaged 8 other “relationships” per year. Compare this to the US average of 67% of heterosexual relationships last more than 10 years.
The gay women seem to be better in this regard but they have their own issues (see statistics on spousal violence).