I think you're right in saying that you're taking it too literally. But to be technical, it says wanted, not preferred and I think it suggests the dc was an accident as opposed to a mistake. Maybe these are popular with those who have an accidental pregnancy.
I wouldn't say I'm 'one of those people' as I don't find humour especially in controversial things, although I do tend to see a funny side to almost everything, including children swearing. Maybe I just find it funny because most of my DC are the result of DH just wanting a shag. We have made jokes about this kind of thing, it often comes up as we have 5 DC and people might joke about not having a telly etc, so when I was pregnant again DH might joke he was only trying to get his leg over.
I just think of it as being a bit like when my DC are running wild and I say to my friend with 1 child to save herself and not have any more. I have been known to come home to the house being a bomb site, DC going manic and comment that they would be the most successful advert for contraception, doesn't mean anything other than a jokey comment.