There's also the whole slut shaming tendency of our society. It's pretty engrained.
Man wants sex. Man posts ad for casual sex. Any woman who responds is obviously a slut, because only sluts want casual sex. Man does not want slut. Man wants "good girl" whom he can convince to have casual sex with him.
Just a theory. Not saying it's a conscious process that people go through. Most slut shaming happens when we're not really paying attention. Also not saying it explains "all" the instances of the contradiction. Realistically speaking, the reason why men post ads seeking "the one" and then send introductions asking for casual sex, is not "a reason" but a multitude of reasons, and probably a combination of several in every instance.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker