just tested the battle, it seems you are winning by flirting BUT you don't have at least 40 lust to mate him, right? that is not unlocking the next step here, try fantasizing to give yourself 40 lust before winning (or just win by HP, that skips any sex tho), i will report this as a bug.
(deciding to not have sex after winning by lust also does that btw, so either HP or lust and have sex to proceed).