Most Helpful Girl
You can't do anything. You should try to forget about her. She already stated that she is not interested in boys, which either means: 1. She is interested in females, 2. She is not interested in dating overall, 3. She is not interested in dating you.
Anyway, you cannot make her change her mind, therefore you should find someone else.
Most Helpful Guy
Couple of problems there. Is she gay or just not interest in dating/marriage at this time?
Are you even dating her. You can't just outright propose marriage without dating someone, that's silly (unless it's for a green card)
And, NEVER, EVER propose on Facebook. That's a NO NO