Among the top concerns concerns couples mention when starting therapy is a complaint that they aren't having enough sex. For heterosexual couples there is a good bit of research and literature that can be used to guide treatment and educate couples about how the frequency of sex changes over the course of the lifespan (i.e. sexual interactions decrease after the birth of a child.)
When gay and lesbian couples bring this question to my office, however, there is very little in the way of guidelines. Research on sex within gay relationships tends to feel almost voyeuristic rather than scientific. There are lots of articles about monogamy and the number of lifetime sexual partners of gays vs. straights. Some of that research is important. One article, for example, studying monogamy and open relationship agreements found that male couples who discussed sex outside the relationship have lower rates of HIV transmission between partners.
The few articles I could find on frequency of sex in gay relationships suggests that couples both gay and straight are having less sex now than they did 25 years ago. 97% per cent of heterosexual females reported having sex with their partner in the last year, but only 93% of gay male couples and 89% of lesbian couples report having sex with their partner in the last year. In 1975, the same figures were 95% for gay males and 98% for lesbians.
My research review still doesn't seem to provide much guidance in terms of what's typical in terms of frequency in LGBT relationships or help explain what causes some of the variance in sexual frequency along the way. I could speculate, but the data to back up my guesses just isn't out there.
Update: Another set of researchers examines the lack of research for same-sex couples: “Is This Normal? Is This Not Normal? There Is No Set Example”: Sexual Health Intervention Preferences of LGBT Youth in Romantic Relationships