You could of used "out of wedlock" it's a nicer term. Well I suppose in today's society you don't have to be married to have a son with some one. Its not neccesarily a bad thing and Im guessing we can't come up with an accurate answer to your question because we are not Letterman and his now wife so we dont know their reasons. Its not really a funny matter either, its kind of making fun of the child. No offence. x

The same reason Trig, Trapp, Tricky or whatever Bristol Palin's kids name is, was born a bastard (your word).