Oh, the Manifesto IS about them as a pairing but I don't think the attractive of Major/Integral is for smut (AlucardxIntegral is probably the best het pairing to visually smut since their design is fabulous). I can give you fic recs in which the ship has been displayed (and not in a sexual way, I think Integral, Alucard and Major have their satisfaction through bloodshed for different reasons, so sex is not really that important).

Eh, they will die, I suppose. Major is one of my favorite vampires, he makes the best villain I have seen in a long time (he won, even if he lost the war and his life, he already won, he started a war the world would never forget, what he wanted).

I like Integral, Pip, Anderson and Seras, in that order as favorite characters.