Just so you know, and I'm not trying to police you here, we aren't allowed to discuss any mods or share links to them, due to the risks and crashes they cause, plus the fact that this is a PG Forum, and 90% of the modifications break the T for Teen Rating.
I think this behavior is normal. However, I think if a Sim has the inappropriate trait, they will share a bed with just about anyone, regardless of age and relationship. If you're okay with cheating, simply try "Modify Traits for Active Sim" change the trait to inappropriate and see what happens.