I highly recommend you pick up the comics. It's a great read. The time waiting between issues is excruciating for me.
But if you want to read the comics for an explanation on the outbreak, you're going to be disappointed.
I don't find the sex in the comics at all distracting, in fact I find it very realistic. The characters are all human, and they have human needs, like companionship and sex. That's why early on in the series, people started pairing off. In a situation as stressful as the End Of The World, wouldn't you want to share some time with someone and get laid in the process? After all, you never know if the next day is going to be your last.