Marriage is spending your life with someone else. Life has it's good and bad, just like marriage. You can decide for yourself what you are willing to do deal with and what you aren't, but "marriage" isn't the issue.

...to a mentally ill person.

Marriage means you share your spouse's life and well-being. Often for better, sometimes for worse.

My mother has often said that “a parent is only ever as happy as their least happy child”. I think the same sentiment holds true for marriages.