Marriage changes passion.
Suddenly you’re in bed with a relative.
Another way to say that is you're in bed with someone you can reasonably assume is involved with you as a person, rather than as a body part, and has a reasonable chance of working out the hazards of life with you and whatever children turn up.
I think that's a lot more *ahem* inspiring than being in bed with someone you're pretty sure doesn't give a flying freep if you're dead tomorrow, as long as he ejaculates tonight.