What do we mean when we say it today?
Nowadays, we're a little less literal with this phrase. We guess it could apply to two strange people who are, ahem, in bed together, but usually it refers to partners or colleagues instead.
Just say your hippie chic friend Ryder and your career-driven, Type-A pal Alex team up on a school project. You might say they are "strange bedfellows" because they are two totally different people working together. It's just another way of saying, "hey, that's an unusual pairing!" It doesn't mean they're dating or anything.