As a parent, your top priority in life will always be making sure that your children are safe. Therefore, you'll always be thinking about potential risks that they may be subject to, and how you can make them safer. If you are not in a relationship with their other parent and you share custody, there will be a certain amount of time in which you will have little to no control over your child's safety. This can feel scary, especially if you do not trust the other parent's ability to keep your child safe.
If you get married and the relationship doesn't work out, you may be at risk of losing a significant amount of your financial assets, particularly if you are the sole breadwinner in the household. This is why it is very common for high-asset individuals to ask their fiancés to sign a prenuptial agreement before marriage. This document should theoretically ensure that one spouse does not walk away from a failed marriage with their ex-spouse's hard-earned assets.