It didn't seem to be working for me, but I tracked the problem down to the fact that I wasn't marked as the victim in Sexlab; it was just returning null when the victim was requested in the FuckEmUp event handler. SLFTU relies on the Sexlab victim identity to decide when to apply the damage effect, so it was never being applied.
Fixed for myself by replacing:
If victim == Game.getPlayer() && Game.getPlayer().HasMagicEffect(SLFTU_SRA) == true
If actorList.find(Game.getPlayer()) >= 0 && Game.getPlayer().HasMagicEffect(SLFTU_SRA) == true
in slftu_main.psc and recompiling. That does change the criterion for applying the effect though. This way, any sex you participate in counts.
A more correct solution is probably to go through all of the dialog options to make sure they correctly set the third parameter of StartSex (the victim) to Game.getPlayer()