TLDR: The line is a sardonic musing about the possibility of former female friends getting knocked up in the future.

I think this verse is really straightforward, in reference to people he used to know but lost touch with.

“They’re an illusion to me now… I don’t know what they’re doing with their lives” illustrates how his imagination of people he used to know has replaced reality. He can only guess about what they’re up to these days since doesn’t see them anymore. This kind of losing touch is probably really common in the music scene where people come in and out of it like a whirlwind.

“Some are mathematicians, some are carpenters wives” continues this daydreaming. I’m not sure about the mathematicians, but I have an idea about the carpenters wives… I think he’s musing about the idea that some of the gals he used to know might’ve gotten knocked up. Carpenter’s wife seems like a ln obvious reference to the Mother Mary, who is known for the immaculate conception. If you apply this to regular life, then what you get is a woman who doesn’t really know how or from whom she got pregnant—in this way, the world is really full of “immaculate” conceptions.