My tendency is to think it is not so good, as far as mental health is concerned. My understanding of the term is that it means casual, no strings attached sex. It usually happens with people who you know, casually, and are attracted to, and don’t mind hanging out with, but have no interest in going any further than having a little fun one night (or maybe more).
The thing is that hook-ups can often turn into imbalances where one person becomes more attached, as a result of the sex. You see, I don’t think you can truly separate sex from emotions. It is not merely having fun (unless it’s a commercial transaction, although even that can be difficult).
Sex is best when it is coupled with strong emotions. But beware! Emotions can lead to an involuntary facial expression, but it works the other way around, too. Voluntary facial expressions can cause you to feel the feeling associated with that expression. In a similar way, having sex with someone you don’t feel very close to can make you feel much more involved with them than you were before you fucked each other.
I.e, you are fooling yourself if you think you can just hook-up with no emotional consequences. There isn’t really any such thing as casual sex with a friend. It is always more, and you can both try to deny it, but you’re just messing with your own heads, so don’t be surprised if things have a way of getting weird and complicated.
Sex is a way of expressing some of our most important and deep emotions. To try to engage in sex while pretending that there’s nothing more than fun involved is to ask for a cognitive dissonance, which leads to confusion and emotional uncertainty. That can be painful, and so hooking up can cause that pain.
On the other hand, we are social creatures, and moral codes aside, I think humans cement strong ties by having sex with people they want to be close to. I think that because sex outside of primary relationships is considered immoral, this leads to a lot of cognitive dissonance, as people try to reconcile their feelings with what is socially acceptable. In any case, if people want to make love to those they love (but not in a permanent partnership way), they have to do it in secret, with shame.
Hooking up is a term that tries to reclaim some acceptability for the practice. But, at the same time, it sets up rules that are contradictory. You want to get together and share body fluids because you care about a person, yet you have to pretend it really isn’t that deep.
I don’t think hooking up is good or bad, but I do think our attitudes about sex are fucked up, in general. I understand that STDs and primary relationships, and taking care of children, and all kinds of other things (social acceptability and status) are tied up in this complicated mix. I think it leads to a lot of emotional pain—more for some of us than for others. I think hooking up is a way to try to deal with these conflicting social mores and personal and emotional needs, but that it is a dysfunctional way of coping. It is pretending to be something that it is not and can not be, because what it is and should be is socially unacceptable.
[edit] I’d like to see you put that into your argument [said ironically]. If you do, let me know how it goes over, lol!