Maya and the girl were making out when Nemu found them; in the process of having sex, but still didn't. When Nemu arrived and left, Maya pushed the girl: he regained his senses by seeing him, because he was clouded by a "broken heart" and just wanted to forgive about it (like when people go get drunk after some ugly shit to forget). And well, that girl looks just like Nemu, giving us a hint that even a "drunk in sadness" Maya subconscionsly still thinks only about bae and his favorite part (his face). So I wouldn't say Maya wanted her, but only needed to scape from the situation.
I actually find it nice to see that he would still choose Nemu over a girl that looks just like him, Maya being "hetero"; it means Nemu goes beyond all that. Nemu is just being insecure about it, but Maya did stop because he was actually not "doing it" because of her, but only to distract himself. Even if he pushed her because of Nemu, still means a lot.
Can someone explain me why Nemu said with Maya that"I thought you couldn't forget that girl and that you were still seeing her" in chap 5 page 23-24.I don't really understand