Thanks for suggestion, actually worked :) I'd tried giving them drink during date pre-sex already, and giving them aphrodisiac during sex, but neither worked. However, turns out giving them drink during the sex does. Not exactly ideal from an RP view, since getting my girls drunk to have sex with them seems a bit off, but better than the alternative. And much as I enjoy RPing my games a bit, not gonna let it get in way of being able to play them.
Definitely something wrong somewhere though, since as you said, shouldn't be encountering this issue in the first place. And even if consent is at 'willing' rather than lusty, still seem to have the issue. Still, at least there's a way around it now.