Well, yes, I suppose that would be it. Or we just have to sort this one into the "suspension of belief" box. ;-)

(Although, in the car scene there was no particular reason for him to get an erection. It didn't seem pre-meditated for them to have sex at that point. In fact, I got the impression they had stopped doing that some time ago, from what Annelise said.)

With Martha we've definitely seen Clark's "sex face" of focused torture - LOL!