I said earlier that I wonder whether the same affliction that's affecting Selene also affects Hecate. If so, I wonder how Pirot King is going to demonstrate that with folks like Mrs. Collins.

TBH, I suspect what's actually been going on isn't what either goddess originally assumed. It's very possibly (probably?) true that they need "adoration" (via transfer of stored-up sex something-something to the goddess... via sex). But to me it seems clear something else in the universe (among deities, at least) is going on.
It's not like the MC has abstained from regular sex — at least, it's sure as hell not true in my own game. Depending on your choices, he could now regularly be having sex with Rin, Molly, Annie, Mrs. Collins, and Victoria. Even if that's not enough to heal her (them both, actually), it's more than they got before the MC made the deal. Theoretically, shouldn't Selene be getting better, not worse? If she already was in severe trouble, you could say she was already too far gone and this is the consequence. But if that were true, I'd have expected she'd have collapsed much earlier in the game. IDK... something doesn't add up.