She means that straight women, like straight men, fantasize about two women together.

Poor thing doesn't get out much if she doesn't know any women who like the guy on guy thing. (she needs to hang out at my place when we watch Torchwood!)

Sadly, I sort of agree with her about the poster. While my first thought when I saw it was, "cool, two chicks", it is horribly "artsy". (Though, that may just be my over-35, less-than-model-perfect self reacting to the underlying message that only skinny, twenty-somethings are sexy.)