If you marry someone who owes back child support you will have to help him pay the back the support. The law states that is you get married to a person who owes back child support, and he/she can't pay it you will have to help him pay it back. However, you can sign up for a pre-nup will that cover me from being responsible for his arrearage.
I believe, once you marry you 'inherit' his bad debts too .. much the same if he owes for building materials at your local hardware store .. It's not like if he leaves you, that you would be held responsible or he can pass his responsibility onto you .. HIS name is still on the paperwork as the one who owes.
You are not liable for someone elses child support. Be aware however, that if back child support is not paid, it can be taken out of any federal income tax refund that person may be getting. If you file a joint return you can claim that your wages are not subject to his support payments (check under innocent spouse). Some states also do this but I am not sure which ones. His wages can also be subject to having child support payments taken out but I do not know how this works.
Dustee is right. This is happening to my former daughter-in-law in Wisconsin. If the person's income who owes child support is reduced because of that, the other person's income has to make up for it in one way or another-whether in the form of having to pay for family bills they usually would not of had to pay, or also those emergency situations that always arise in families . Plus child support can go up but income may not. It's a mess. Everything needs to be layed out in perfect order before marriage. Perhaps contact an attorney? Better to pay one now than later. I wish you good luck.