This isn't about married guys or girls posting ads. This is about site like YAHOO!, and match.com, posting fake ad themselves to bolster their number and make you want to join. I don't know about the others, but depending on the age range of your matches, they will show you lots of ads from (for me) girls within that range.
I see a lot of ads with the typical background info, and usually ANY as the preferences in all catagories. Only one pic. This is the biggest hint. Some stock photo or model. My experience it has been 23yr old women, with one photo, and preferring guy 18 to 40. ANY race, ANY height, etc. Wouldn't be strange but there are literally hundreds of them just like this with only a slight change in the ABOUT ME section. Otherwise a different pic is all that they have.
Check it out for yourself. Use the free profile set-up, and the two week free trial. Be weary if you join. They are LOTS of legitimate profiles, but it has to be illegal or immoral to make you believe these other people are on there when they are not.