Folic acid just prevents birth defects. Most people, including you, don't get enough folic acid through food, so it's a good idea to take them.
You need to have a conversation with your girlfriend about your fears about having children. You should be in agreement about this before you marry.
The fact that you are thinking of replacing her tablets without telling her tells me that, at a minimum, you two aren't communicating enough. It also means that you have a tendency towards being controlling, which can lead to your being abusive. You need to put a stop to these tendencies right now, before you marry. It's completely unfair to her if you do things without telling her and without her consent.
Don't depend on her to use birth control, use condoms with spermicide included every single time you have sex. That will tell her clearly that you're not ready to have children.
If telling her you're not ready to have children is too much for the relationship, then don't get married.