They may be sold to a different owner who uses different standards or may engage in some of those shady practices to boost membership and use. You simply have to watch your back on any online dating site.
To answer your question: it depends on what you mean by legitimate.
My first recommendation would be to try Lavalife's Intimate Encounters section; you sign up with them and then decide which of the three sections you want to be a part of (Dating, Relationships or Intimate Encounters).
You can use different handles in each section under the same account too, so no one will be any wiser if you want to advertise for one thing in one place without ruining your chances for something more should it come up.
The best way to go about things is to take advantage of the free registration and trial period most premium dating sites offer to check the site out and see if free online dating is something worth pursuing altogether.
You can experiement with some of the best free dating sites, without opening up your wallet.
Profiles that haven’t been active in years are still displayed in search results, causing you to waste a lot of time just going over them.