" is it because men really really really have a thing about lesbians"?
As a lesbian, I think there are several possible reasons.
Men in general seem to be puzzled by lesbians (gay men included!), because many of them seem to find sex without a penis incomprehensible! I kid you not, it is an attitude I personally have come across from gay men, and supposedly intelligent, educated, informed, politically active gay men at that! So, from Casanova onwards, penis-free sex was somehow seen as not real sex, and usual sense of male entitlement ends up with it as the bit of titillation before manly-man steps in for the REAL event. Hence the prevalence of supposed lesbian sex in straight male-porn.
Okay, so you could see not just male-constructed femininity (make-up etc, and also including porny breast implants and hair extensions etc etc), but male ideas about 'lesbians' ALL being incorporated, either in a voyeuristic 'I watch others', OR in an autogynephiliac 'I do it TOO' trans way.
Frankly, the whole cotton ceiling is all much of a muchness, from trans who get upset because women don't want them in the ladies, or the female changing rooms in your local department store, or the female locker rooms and sauna at your local sports club, to getting upset because 'real' lesbians don't want to join in their pretend lesbian sex games either. Overall, it's the SAME female refusal to accept them into what they see as the restricted magic lady-gardens of TRUE womanhood..............