IMO if the player declares that the PC is engaging in sex but does not declare that they’re engaging in unprotected procreative sex, a DM shouldn’t presume it when deciding to resolve the action.It's up to the father what he does.
If they say the bolded ahead of time in a future or present-tense manner, of course I'm fine with it. Afterwards, though, it's too late - retcons to declared-and-resolved actions are never an option be it in this area or any other.