(contactmusic.com) - Madonna and Guy Ritchie's marriage will fail if they adopt another child, according to the film director's father. The Material girl is reportedly looking to adopt a baby girl to add to her three-strong brood, but Ritchie's father JOHN believes the extra pressure could seriously threaten the couple's relationship. John insists son Guy is a perfect father to Lourdes, ten, Rocco, six, and Malawian David Banda, one - but admits the couple are going through a difficult patch. He says, "They've had a difficult time. (David) is beautiful and Guy is very good with him. But I don't think he'd want to adopt again."