If I just consider the joke itself, the punchline really comes from the fact that Sanda seemed like the cool senpai (she even smokes) and she was even completely dignified while proposing for kouhai to move in with her. (Also I just now realized that kouhai's name is a mystery. Weird.)
But if we look for the justification of the joke, as you did, it can also be considered a more subtle implication of the atmosphere where society assumes queer people don't exist, than the "but we're both girls" line. Sanda may have let her guard down in this sense because she didn't consider the romantic implications, as she was talking to a girl.
It would be refreshing if this weren't the reason why she didn't realize the implications of her proposition, but we have no evidence for that. For example, she didn't seem socially inept in any of the previous panels. But I also prefer the subconscious romantic feelings interpretation, even if it takes a bit more reasoning, as the story is right now.
I would like to see more from these two. Mochi often touches on previously unexplored niche territories within the yuri genre, and this is one of those cases.
You make a valid point. I complete failed to acknowledge the societal ideologies Sanda might be influenced by. I guess my own preferences simply blocked the ideology of "queer people don't exist" because I'm of the ideology that everyone should be gay. Well, that everyone should be female and gay, but then there's the situation of how are they to reproduce. There are some ideas that come to mind, but this isn't the place for that i suppose. :P
