That does not sound familiar, and I reread Rise and Fall of a Dragon King just one year ago.
Sorry, it was Rauri the Half-Elf, one of the Quarite druids (not a templar) who catches Hamanu’s eye, and Hamanu visits him in the night as a beautiful woman and leaves him flustered over the mysterious “yellow eyed woman” and with what Pavek recognizes as Hamanu’s shirt.
Hamanu may have sent those events into motion, but it doesn’t seem to me that Ruari had sex with Hamanu, especially because Pavek has a vision of this woman and Ruari together, with children, in the future. This is also back in TSR’s code of conduct days.