Fellatio on its own is not actual consumption of an animal product because the penis is not being eaten, so by that factor it could not be rightly called un-vegan. Now, when it comes to the swallowing of semen, that would be the consumption of an animal product, so I suppose that could be called un-vegan. However, considering the production and consumption of that animal product (semen) is done with full consent from a very willing animal source (the guy) rather than it being taken without permission, that would not violate the tenets of veganism which shuns the taking and consumption of any animal products, including things like honey or milk which cause no harm to the animals they come from. The same goes for cunnilingus performed on a female, considering the sexual fluids she produces would be invariably swallowed as well.
And keep in mind, a baby breastfeeding from a mother similarly violates the tenets of veganism, but nobody with more than two brain cells objects to that.