Your friend has probably heard that from another friend.
Do yourself a big favour and try one of them out, make your own opinion based on facts… Obviously there are going to be some strange folk on these types of sites, but on the paid adult sites, it’s seriously such a small percentage that it’s not even worth worrying about.
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.
After Lavalife, I only know of a few others that are worth mentioning, although I've yet to find one that's truly ideal.
Always be wary of the signs of an internet dating scam.
Good sites can go bad, and those I don't recommend may improve.
To answer your question: it depends on what you mean by legitimate.
Are you looking for an adult dating site that caters to adult encounters or casual sex, that isn't full of scammers or fake profiles?
There are a lot of adult dating sites that don't have a lot of users, or use sneaky marketing tactics like the bait and switch you're referring to.
We've all seen them - the ads show a bevy of beautiful women in your area, but when you sign up, you realize that the women were just ads that were geographically targeted to your search preferences.