I don't know if your partner is a man or a woman so I will just refer to "he" in my answer.
He'd probably call it "keeping his options open". If he is on dating sites then he intends to get in touch with other people. What he does when he gets in touch with them is the crux of the matter.
Is he just window-shopping? I must admit I've *looked* at dating sites in the past, but purely through curiosity. I can appreciate an attractive bloke as much as blokes appreciate attractive women. The difference is that I would never sign up or try to make contact with one of these men.
If he has made contact with any people, what has followed? Has he sent flirty or sexy messages, or just chatted with them? If he's just chatting, does he feel there are things that he can't open up about to you, his partner?
Does he have many friends in the real world or are most of his friends online? If online, he may purely see this as way of extending his friendship group (and therefore his Facebook friends list and his Twitter following if applicable).
I would consider it cheating if he has been in contact with any of these people for sexual purposes, whether purely online, by text message, on the phone or in person.
He obviously knows that you know he's on these sites. How long have you been together? If you have only been seeing eachother for a matter of weeks he may be looking for his next conquest. If you've been together ages he may be a little bored - don't be offended, it happens on both sides and can be sorted in a variety of different ways (there's plenty of websites to help you with making your relationship more exciting, and not just in a sexual way).
At the end of the day, people will always meet other people on the internet and flirt - be it through Facebook, World of Warcraft, forums for their favourite bands or dating sites. It's what he actually *does* with those people he meets that makes the difference.
Are you entirely happy in the relationship? If not, you could use his membership of these sites as a good reason to get out while still making it his fault (believe me, this makes a difference!).
I personally would not be happy if my boyfriend was on dating sites, but then I know he uses porn on the internet and makes contact with porn models. I can't stop that, but I do try not to think about it. We have a pretty good relationship so I ignore it (just like you ignore those niggly things that partners do which really annoy you).