Adoption visitation contract: can it be upheld in event of divorce?

Someone would have to review the contract language to be certain, but I think it's very, very unlikely that your divorce would end the visitation contract.  This isn't the biological mother's fault, so there's no equitable reason to punish her for the failure of your marriage.  Perhaps more importantly, from a legal standpoint, is the fact that she isn't a party to your divorce, and I'm assuming that your divorce papers aren't asking the court to terminate the visitation contract.  If you did put this into your divorce, now, you'd have to notify the biological mother and give her an opportunity to oppose that.