you cannot get marriage visa without proof of legally married to a Thai woman. If you visit Thailand to marry her, you cannot get non-O before your marriage.
Just unnecessary additional info as many people are mixing up visa validity and entry stamp.
As Lloyd said, the visa itself's validity of 90 days (<- the period you can use that visa to enter Thailand to get an entry stamp for the status on the visa) starts on the issued date but the Thai embassy/consulate.
Your 90-day stay on Non-O starts on the day you enter Thailand within the visa validity as day 1, even if you enter Thailand on the last day of visa validity.