Rey thinks Finn and Poe should be together but she has weird ideas of things that are romantic. Like, she points out physical attributes of both of them that would be "good at surviving" in sparse conditions and how their inability to to get pregnant is a blessing during tough times. Or how "he didn't attack and maim you for stealing his food" is a sure sign of interest/love. Poe and Finn are too nice to tell her that one, her perspective on this is a little skewed, and two, they're already together.