On lesbian obsessions with straight women--everyone wants what they can't have. There's a strand of obsession with straight men among gay men, there's definitely a thing among some straight males for lesbian women although it's less an obsession than a privileged "How dare they not want us?" thing. Dunno if there's really a thing among straight women for gay men (except voyeuristically as in yaoi) but it's widely agreed that there's a thing among many straight women for attached men, which would also be "wanting what you (probably) can't have".
And of course from the perspective of any individual lesbian or gay male, there's no obvious difference between a straight and someone in the closet. The more repressed the society, the more you basically have to aim for "apparently straight" people because that's about all there is. Japan being a fairly repressed/repressive society in terms of gays, apparently straight people is about all you can target unless you're in the Morishima Akiko version of the universe.
So it's not like this is some weird unique inexplicable thing about lesbians or lesbian culture.