“Lesbian bed death” is the term used to describe the steep decline in sexual activity that is thought to occur in long-term lesbian relationships.
For the record, research has found that on average, lesbians do tend to have less frequent sex in long-term relationships than other couples (opposite sex and male same-sex relationships). However, a decline in frequency of sex occurs in pretty much all types of long-term relationships.
Additionally, it is also found that on average, women in same-sex relationships have LONGER durations of sexual activity (avg. 30-45 mins), compared to other types of couples (avg. 15-30 mins). And levels of sexual satisfaction in lesbian relationships are just as high on average as in opposite sex, and male same-sex relationships.
So they may fuck less, but when they do get down to it, they really put in the time and effort.