Parents who are divorced need to budget for those payments. It doesn't matter which side of the matter you are on. If you are making those payments, you must ensure that the money is available. If you are the recipient, you should set your budget to use them as a windfall just in case they are late.
One of the best things that you can do when you are planning your wedding is to discuss a prenuptial agreement with your future spouse. Many people tend to think of prenups as a negative component of the engagement and marriage. But there isn't anything negative about wanting to protect yourself and the other person.