They’re just creating a scene for Mongo to interrupt. The full dialogue lasts just long enough to create a friendly atmosphere in the bar - it’s something like
Guy 1: “Gentlemen, did you know that even now, in France, Louis Pasteur is working on a vaccine that will cure anthrax? Just think of it- the end of hoof and mouth disease!”
Guy 2: “Nevermind that shit, here comes Mongo!”
Then Mongo wrecks up the place. I’ve found Guy #2’s line very useful in my daily life.
I’ve always interpreted that as “not wanting to be caught dead in long underwear”. They’re high society gals & should be thought of as only wearing silk frillies, not woolen long johns.
I assumed that gestating Satan’s baby is just a bitch. What with the horns and so forth…
Then again, I do remember the neighbor lady (Mrs…Cassavetes?) giving Rosemary the special drink every week. But I never thought that was the cause of it. Because she stops taking and then retakes it, but the pains never come back.
It did make for a good plot device. I guess I’ll just go on assuming it was the baby.
When she starts taking the drink again, she also gets a little cake of something like marzipan to eat with it, which could be a device to negate the pain.
I think this is right. I think she’s saying “I don’t care what your intentions are, we’re wearing long underwear (that we don’t want anybody to see if we get in an accident), so be careful!”.
According to an interview I heard with Mel Brooks, it was a riff on something apparently common to westerns at the time. To wit: Things added in to point out the ‘historical significance’ of the time. The mention of Louis Pasteur, out of the blue, was his stab at the various ‘out of the blue’ mentions of other historical things he saw / heard in movies and found vaguely ridiculous.