Possible reason Nanao gave sensei a blow job could be because they're close friends and this was like Nanao's farewell. But both of them are aware of sensei's feelings for him and since sensei really is his only close friend, Nanao's actually doesn't want to lose his only friend. It's possible sensei coerced Nanao into giving him a blowjob as a "farewell gift", but honestly, rather than Nanao just playing around, it really did seem like it took a lit from Nanao to pleasure sensei. He was shaken and disoriented when he was found by Aikawa, so it couldn't have been something lighthearted or easy for him.
Honestly, it wasn't denied in the end that Nanao was a very loose person. He didn't make the first move most of the times, and more often he refuses propositions made to him, but he's guilty of having one-night stands outside of university, and a one night stand with his closest friend, a professor. That last bit is already enough to tarnish an ordinary person's rep. It's amazing he's not bullied at all, come to think of it.
But Aikawa still wanted him and accepted his past. His *past*. Meaning it's expected he won't do anything shitty while they're together. So I think there's no need to be harsh on Nanao as past is in the past. He only deserves to be hated or replaced by someone better if he actually ends up betraying aikawa while they're together. Cuz that proves he really is a worthless person.
Possible reason Nanao gave sensei a blow job could be because they're close friends and this was like Nanao's farewell. But both of them are aware of sensei's feelings for him and since sensei really is his only close friend, Nanao's actually doesn't want to lose his only friend. It's possible sensei coerced Nanao into giving him a blowjob as a "farewell gift", but honestly, rather than Nanao just playing around, it really did seem like it took a lit from Nanao to pleasure sensei. He was shaken and disoriented when he was found by Aikawa, so it couldn't have been something lighthearted or easy for him.
Honestly, it wasn't denied in the end that Nanao was a very loose person. He didn't make the first move most of the times, and more often he refuses propositions made to him, but he's guilty of having one-night stands outside of university, and a one night stand with his closest friend, a professor. That last bit is already enough to tarnish an ordinary person's rep. It's amazing he's not bullied at all, come to think of it.
But Aikawa still wanted him and accepted his past. His *past*. Meaning it's expected he won't do anything shitty while they're together. So I think there's no need to be harsh on Nanao as past is in the past. He only deserves to be hated or replaced by someone better if he actually ends up betraying aikawa while they're together. Cuz that proves he really is a worthless person.