For some people, the claim that there is something serious or significant about sex will seem obviously true. For others, the alleged truth of this claim is not so clear. One contemporary philosopher has offered up the notion that perhaps we ought to see sexual exclusivity and romantic love as separate entities. One consequence of this is that we could love one person and be committed to that relationship, but have merely sexual relationships with many other people as well. The idea is that we should view sex as we currently view sharing a meal. Neither is that big of a deal.
It is clear that some people opt for this sort of approach to sexuality and love, but the deeper question is whether or not it is wise to do so. It is unwise to view sex too casually. Having sex is unlike sharing a meal with another person in many ways, and some of them are morally and psychologically significant.
To see why this is the case, consider the following argument from Matthew Flanagan:
This is an interesting argument, because it brings out the seriousness and significance of sex (see the comments at MandM if you're interested in some critical discussion of the argument; the argument needs a bit of work but it makes the point that it is intended to make, I think).
Of course sex is fun and pleasurable, but it is also a serious part of human life given human nature. Sex means something to us, something more than mere pleasure, or at least it should. If sex were merely a casual recreational activity, then the above argument wouldn't work. But I think the argument ultimately does work and that we ought to conclude that even though some approach it as such, sex is not mere recreation. It is something more, whether we recognize that fact or not.