Actually, now that I think of it, the problem in case of a male child would not be how to provide for him, but how to disinherit him - or rather, have him declared not to be his heir as a legitimately born child would automatically be considered as legally his. I don't exactly remember how it came out in the open that his wife married him already pregnant with another man's child, how much of it hinged on the baby's appearance and how much on the wife's admission, which she might not have done if she had borne him an heir. And could a nobleman really disinherit the heir of his title based on appearance only? Seems unlikely. Lydia being a girl saved everyone a horrible load of trouble, and she was fortunate enough to have Adam for a stepfather.