I don't think they've established how devils handle breach of contract in this universe. Sou's contract with Kogure is that she take his virginity, right? So, hypothetically speaking, if Sou manages to convince Seira to loosen up or lose his virginity in some other, Kogure-free way, what would happen? Would he be stuck with Kogure forever? Would she have to eat him? Rape the girl he slept with and take Sou's virginity second-hand?
I think that Kogure was just supposed to have sex with him, and that Sou loosing his virginity the consequence of their contract. But if the contract does actually specifically say "virginity", then it will be interesting to see what would happen.
He was summoning a devil to specifically have sex with it, didn't he? "I would lose my virginity with an alien, an angel or a devil!" was what he said. So it must be Kogure and Sou. May be contract will be just considered void.