As a straight man I find myself not turned on at all by lesbians. As a teen I was, mainly because there was a pro-lesbian sub-text in much of the soft porn I was exposed to.
In my late teens and early twenties I got to know a few lesbians, with getting to know them came the realization that things are only like porno's when someone's getting paid. Neither lesbian sex nor gay sex are ever as they are portrayed in movies.
Sometime later I came to the realization that straight sex is never as it's portrayed in movies.
How is this relevant to the topic in hand?
Men get turned on by lesbians much more than women get turned on by gay men
Men get turned on by the image of lesbians as presented in porn much more than women get turned on by the image of gay men as presented in porn.
Looked at this way the reason is simple --- the porn industry's ideas of lesbians and gay men is perverted by the gender of the overwhelming majority of it's customers.
Most porn is created for men.
Much porn apparently created for women is created for men and disguised as porn for women to increase the men's voyeristic pleasure.