In Colorado, has there ever been a law that prevented me, a natural male, from entering the women-only restroom for the purpose of using it?

There was never any law on the books stating that a man had to use the men's restroom or vice versa. Otherwise thousands of women would have been arrested as we all know women use the men's rooms at concerts, bars, etc if the line is too long for the ladies room.

The statute of limitations dun be gone gone, dude. Butcha can look up de superseded code and legislative history yourself, if you care.