1. Ordinary people are food. It's not gay to #### your food. What did you have for breakfast this morning?
2. Vampires can be very old. In olden days it was easier to be gay because the dumb ####s with pitchforks were too ignorant to know that gay was a thing that they had to stop. Less resistance vs. instincts.
3. Satanism, that isn't so hand in hand with Vampirism anymore, requires a variety of non-boring sexual acts to pray to Satan. If you're willing to #### a goat, same sex ####ing is a breeze, surely?
4. Control. The head vampire/Cult leader asserts dominance on underlings by ####ing them. It's not gay for a guy to #### another guy if the first guy is only trying to break the second guys spirit/will to avoid a knife fight over a pretty lady further along upstream time. (It is gay however if you enjoy yourself while performing gay sex.)
5. Erections are a waste of blood. Non gorged vampires don't have the blood to spare to get it up. Therefore lesbian sex is easier, and gay bloke sex is easier if they can take turns (or flip a coin to see who is) having to force the situation (by eating three people) to summon all the blood they need to hoist their mainsail completely.
6. Some lore makes blood out to be something between booze and heroin for Vampires. People on Heroin don't really care about what they have to do to get off, therefore a vampire swollen on blood would have as much the same a devil may care attitude to selecting sexual partners as any other junkie.
7. Vampires have no souls in some lore, therefore they are lacking valuable elements to their identity as persons, that there current needs are not dragged down by a thousand life times of being a human of one gender or another with all the expectations imprinted on them therein. Do souls have a gender, therefore can a boy soul be in girl body or vices versus, or are souls genderless which would would calm the #### down the maleness or femaleness of the flesh craving sex at any cost?
8. After 500 boring years of trying to tap xeroxes of the same dimwitted teenagers, a Vampire might be a little curious if the grass is greener on the other side of the fence?
9. Back to food. Bloodtype is more important than gender, or if the lore is that vampires eat souls using blood as a medium, gender still doesn't count if souls are genderless, and honestly, if you're planning on killing someone and turning them into poo, it's actually rude not to give them an orgasm first.
10. How many brides did Dracula have? 3 and Lucy, means 4? More probably too. I know Big Love makes a strong case for Polygamy, but after a thousand years being henpecked by those nightmare ghouls (although Giles (from Buffy) quite liked them), you can bet that Vlad was gagging for some male company.