Sex during Free Time wouldn't make sense in some contexts. If a girl got, say, beaten to near-death exploring the catacombs, then it would be strange if she went out and had sex almost immediately after. It might have to be based on multiple stats, something like...
Libido > 75
Health > 90
Tiredness < 10
...and then it'd have a chance to happen. There'd probably be some other requirements if the end goal is ~realism~, but that's probably the one that stands out the most to me.
I'd personally like a Rest job if all these changes are going to be made to Free Time. Rest could be just the "raising health, lowering tiredness" function of Free Time. That way, there would still be a completely safe recovery option for people who just want a girl to not die, and Free Time could be expanded with as many random events (negative or positive) as anyone wants.