Sure sex should be taken seriously in a relationship as a way of sharing love. But there are times that sex can be totally animalistic. That is sex just for the sake of having sex. Hey, why not? :)
I guess it all depends on how one looks at sex...some, myself included, see it as something you only do with the one you love, whether it's making love or having sex with them...while others see nothing wrong with having sex with anyone who's willing...and that can be when they're not in a relationship, or when they are in one. IMO, if one looks at sex as 'just for the sake having sex' I'd wonder if they're the type who would have sex outside of their relationship because 'it doesn't mean anything, it's just sex just for the sake of having it'.