Without giving any examples, I can only guess.

From what I've seen, corn is being used as a placeholder for porn because they rhyme.

If you mean something else... maybe give us more to go on. Without anything to go on... that's all I got. I haven't heard any others. Except, of course, puns like you see in the comments, but I think that's just based on you saying corn and them not having anything to go on, so they make puns. I'm not very good at them, and I feel like the best one (amaizing, for maize) has already been taken, so I'm not going to repeat or rework it.