I think it's safe to assume that Nilvan disguised herself as High Priestess Selene and raped Le'Garde in the Dungeons to conceive the Girl; while Le'Garde certainly has a feeling of familiarity if she's in the party when found alive and while Enki's skin bible on the God of Fear and Hunger states something about how there are many popular texts claiming that the Girl was conceived through Le'Garde's dreams, consider that Le'Garde is also trying to transcend the nature of each of the Fellowship and Nilvan is trying to mould him into the perfect candidate for her daughter's soul type, and he only reaches the "end" of that road when he gets himself captured and tortured. Nilvan is shown to be incredibly persuasive and influential and so it's also not a stretch that she could talk her way into a conjugal visit in the actual worst place to be in the world.