I don't understand the joke behind "opposite of seme" scene. Can you please explain it to me?
This is a reference to yaoi sex positions. Seme is the Japanese term for the dominant partner (usually on top), while the uke ("opposite" of seme) is the submissive partner, usually on the bottom.
This is also a pun in Japanese. Seme (same kanji) is another word for "offense" or "attacker". The opposite of which would be "defense".
So, I'm guessing the joke here is basically that Kaoru is perhaps "excessively" normal, since she is so pure she isn't even aware of yaoi sex terms.