Oh feh, the fan boys have been having the hots for comic women since the early days of Wonder Woman and Lois Lane. I mean, who was DRAWING those completely unreal bodies? (which also explains why the male supers were highly muscled eunuchs) . It's not right, but it's fairly common. Practically a comic tradition.
Oh -- and Dora is doing this IN FRONT OF Marten. No hiding, no lying, no subterfuge, no pretending it didn't happen -- lest anyone conveniently forget what the whole stupid Dora/Marten argument was REALLY about. And if he's like most guys I've met, the idea of his girlfriend in a little girl-on-girl while he's watching is at least fun in theory. We know Marten had at least one dream about Faye with Dora, so he's not immune to it (it wasn't a nightmare until he dreamed he woke up as a girl).