What does this joke mean?

This man who was bored of his wife had a mistress. He had with the mistress and a few weeks later his mistress told him that she was pregnant. before that, he already decided to remain with his wife and leave his mistress. feeling guilty, he told his mistress saying "send me a postcard when the baby is born and i wll send you money" the mistress asked "what must i write onthe postcard?" the man replied "ummmm, just write spaghetti" mistress:"ok" about 9 months later, the man's real wife went to him and say " hey honey, this really weird postcard is for you" the husband snatched it over and fainted after reading it. the wife read and was puzzled as all she saw on the postcrd was "spaghetti, spaghetti, spaghetti, two with meat and one without. "