It is forced to abide the centuries, consuming other life so that it may prolong its own unnatural existence. A vampire is a creature that exists along the margins of society, which feeds upon a populous of which it can never, after its mortal death, be a part. It is this marginalization of the vampire that is important, as it removes any human social boundaries. Because the vampire is a creature that exists along a border of life and death, human and monster, there is no reason for a vampire to obey traditional gender roles followed by those in the mainstream. Social constructs such as sexuality cease to be of such importance when the possessor of that sexuality, more importantly than defying ideas of what sexuality ought to be, defies the very laws of life and death.
Vampires are able to eroticize whomever they desire, but that desire can be indirect.
For example, in Dracula, Dracula really only converts women to vampirism, but it is not women that he actually desires. He desires men, and gets to them trough the draining of the women's blood. Dracula is gaining access to the men in two ways through the women. First, with Lucy, he was able to consume their blood through Lucy, where it mingled via the transfusions. He has already tasted the blood of the men, and it is their blood that he wants more of.