When the whole world is getting ravaged in a zombie apocaplypse, wouldn't you want to ravage somebody too?
When you're holding up in some abandoned building with a few other survivors, and one of them looks pretty attractive, it would be really hard not to have sex. With the world about to end, I think it is probably a human impulse to procreate. No way to really tell, but I think its a safe assumption based on our instinct to survive.
Would you even bother using contraception? It depends. It would be really hard having a baby in your tummy and trying to run from a pack of beasts trying to eat it out of you. At the same time, if you don't think your going to last more than 9 months, then screw it.
At this point, some of you may be thinking, would zombies have sex? I think the answer would be no. They lose all instincts other than the desire to feed. Since their bodies deteriorate much more slowly, they can survive much longer than a regular human being can. That being said, in popular fiction zombie muscles don't repair themselves, rendering zombie hordes unsustainable. Because zombies only have the desire to feed, they wouldn't reproduce. Therefore they rely solely on human reproduction to survive. In this way the zombie-human relationship is symbiotic, meaning zombies need a human population to feed on (and infect) in order to carry on their species.