On "The Science of Superman", they try to explain how Superman, traveling an estimated 100,000+ mph, could catch Lois falling from the helicopter at terminal velocity and NOT kill her. I'm thinking that since he can take a huge amount of force hitting him at one time, that his body simply absorbed the energy of the two colliding, and thus I'm thinking if they were doing the Horizontal Injustice that he would find a way to release his muscular energy in such a way that she wouldn't find it any different than a normal earth male. He apparently has some control over the gravity around him (in order to fly) so he could have himself suspended a tiiiiiny amount above her and the bed and float his body in such a way as to not , ahem, arouse suspicion.
I'm also thinking he could probably control any discharge so as to not decapitate his lady friend. That's a hard Hallmark card to find for the saddened parents.