Plutarch raised a similar question and now I'm curious...
Without an explanation for the lower life expectancy of lesbians, the findings would suggest to me that the study is seriously flawed. Off the top of my head, I would hypothesize that the error is due to generational differences in 'coming out' and (more specifically) having one's obituary placed in the 'gay press'..
Public acknowledgment of homosexuality is a relatively new phenomenon, so in other words the 'out' homosexuals skew younger than the actual median age of all homosexuals, and therefore the identifiable obituaries from the 'gay press' skew younger than the true median age of death as well.
That's my guess.
I've read that lesbians have higher rates of other STDs as well as alcohol and drug abuse, and they are not a great deal more monogamous than male homosexuals.