Humans handle pain and trauma with sex. It's cathartic and bonds you with someone.

You know Abraham and Rosalie? They would totally exist in any future hellscape. Billions of people have sex everyday and we convince ourselves it's for many different reasons, but ultimately the drive is caused by the need to further our species.

Sex in the apocalypse wouldn't be nearly as romantic or ideal as anyone's fantasy, but all signs point that if you can fit five minutes in to clear your head and do something pleasant you'll do it regardless of zombies or not.

The downside is random babies will get people killed, but you aren't thinking of that after a day of adrenaline and guts. Your thinking Rosalie just washed and damn does she look good. Zombies are tuned out for at least 3 minutes.