I think it's best not to generalize about the people on dating websites, since each site is very different. I'm sure that there are slutty and non-slutty people alike on there. I've been on dating websites during times when I felt like I just wasn't meeting the right kind of people in bars. If I were a slutty woman, I'd just go to a bar because it's far easier than filling out an online profile.
My understanding with a site like eharmony is that there are quite alot of forms to fill out. To weed out the people just looking for an easy lay, I'm assuming.