Wait, the nerdy gal had such a history? Painfully realistic
EDIT: removing my comment - after getting some coffee, I realized it was pretty rude of me - I'm sorry. Your feelings are your feelings, and you're entitled to them, without some anon putting you on blast on the internets.
That said, I did like how this manga sort of turned expectations on their heads - the sultry, lusty neighbor sensei is actually inexperienced, and is trapped in a vanilla, borderline unsatisfying sex life (didn't she blow out husbando's back and he told her to calm her tits down?). The nerdy introvert manga girl had no problem getting a man and getting the passion she wanted in her past relationships.
I guess it's just disappointing that the whole double standard of "he's a player" / "she's a slut" still exists, and in many manga is totally perpetuated. IDGI, and maybe I grew up too free-love hippie-dippie, but I don't get how a woman would be diminished by her sexual past where a man wouldn't. Or why it would matter for any relationship you were having in the present. I get if it's something or someone specific in their past ("you fucked LUMBERGH?!"), but as long as no one's trading STDs or forming babby, don't let it bother you? There's a million better things to worry about in life... enjoy being with your partner in the present - but just my opinion, I guess.