I personally found the last scene to be either a surreptitious or an unintended confession by Sanae, and her liking guys as just an impulse reply, at least considering the evolution of their relationship.
Sanae starts off only embarrassed by, and distrustful of, Kurahashi's attention, ironically saying hurtful things to her. Her parents' arguments keep reminding her of Kurahashi, what she must feel like not having that in her life. Finally, when the hurting gets turned on her, we find out she was lying about having thrown away Kurahashi's key.
Kurahashi's overt use of alcohol helps loosen Sanae's inhibitions and we learn, alongside her, that her curiosity was tinged with physical attraction; she admits to herself that she wants to see her body, and is the one to initiate contact.
Upon returning home the next day, Kurahashi's still fills her mind; even when her dad promises to give her pointers to make any man hers, the first thing that comes to her mind is Kurahashi.
Back at school, we see they are much closer, with Sanae accepting Kurahashi's request only placing limits for a practical reason. When their conversation turns into a heart-to-heart. Sanae reassures her that they will both find a special person, not man. Sanae's closing monologue also hints at it, how they will come to accept themselves.
All in all, I found this a wonderful coming of age story; we get to watch Sanae mature and become aware of others', or at least Kurahashi's, situation to the point of saying what's needed to show support or clear a bad mood.