Tutoiement and vouvoiement in this film

I caught a screening of The Mother and the Whore in Vancouver this week and was wondering about the use of "tu" and "vous" by Alexandre and the other characters.

Even though they are living together and sleeping together, Marie and Alexandre address each other as "vous", as do Veronika and Alexandre. But Marie and Veronika tutoie each other, even though they've just met. Is that normal in France, to use the formal vous for someone of the opposite sex even when there is an intimate relationship? Or is it just supposed to be an idiosyncrasy of Alexandre's style? And if this was typical usage in France in 1973, has it changed since then?