One of my favorite jokes of all time (I was reminded by the father talking to his children about marriage with their mother) is a little dated, perhaps slightly politically incorrect. Oh, well ...
A couple approaching their 40th wedding anniversary had four sons, none of whom ever made much effort, or spent much money, for the parents. It seems that the sons were always inviting themselves to dinner and pleading poverty, even tho they all had well paying jobs. The sons did, however, offer to take Mom and Dad out to dinner for their anniversary. Maybe things would be better.
The parents were disappointed at the choice of restaurants but didn't want to appear ungracious. The selection didn't seem very good, and when Mom wanted to order lobster, the sons talked her into a different (cheaper) dish. The father started to order champagne to celebrate, but was likewise talked into a cheaper brand. When the cheap champagne arrived, everyone was ready for a toast, when the father had a surprise for his sons.
"I think that you are entitled to know after all these years. You may wonder why you've never seen pictures of our wedding. You see, sons ... you're mother and I never got married. We kept putting it off, then we found out Roger was going to be born, after that, we were too busy or too embarrassed. So, sorry, but we aren't really married, after all."
The sons gasped in horror, and one of them, "Dad ... are you saying that we're all ... bastards?"
The father deadpanned, "Yup. And CHEAP ones at that!"