Dogs will like to lick things that they think tastes interesting, probably cause it's fun, probably to show affection, and probably cause it gives them information about who or what they're licking. Go for a run and get sweaty, and your dog will probably lick you as intensely whether you're a guy or a girl.
With that said, from my experience I'm 100% sure that my partner is aware that licking my vagina is a sexual thing. When he's horny, it's ALWAYS the first thing he'll want to do. He never licks me anywhere else on my body the way he eats me out, and he really really loves it too. I know it's natural behavior for male dogs to do that to female dogs to get the female in the mood to let him have sex with her, so I think that behavior just carries on with a human partner? It'd be nice if he licked other parts of my body during foreplay the same way he eats me out... but meh.
If someone on ZV has an answer, it'll probably be
@allyfitz . She's a big brained scientist.