Like jameany said, it's not consentual. MC can't give his consent because he's still a minor. Now technically you could argue there's no problem as everyone involved is, at least in mind, an adult, but none of the three parties other than the MC know that he's an adult. They should think he's a minor, yet they're fine with this as if they never heard of Age of Consent.
This could be approached from a few angles, such as some textbox that has the MC mention how it's strange they're fine with it, but maybe coming to the conclusion that back in the 80's people didn't care as much (assuming that's actually the case). Just a mention somewhere about how strange this whole event was would've been appreciated by me, because it just feels like these people were made to act like complete aliens in order for the plot to progress in some way.
What's even more baffling is that the guy that invited MC is, from what we've seen when he dealt with those delinquents, actually quite familiar with the law & consequences. Why would he of all people think this is a good idea? Were the MC an actual 16yo in mind, it's plenty obvious that this guy is abusing his recently earned status as mentor to get MC in this situation. Everything about the situation just screams emotional abuse, except for that little detail of the MC's actual age, that none of the parties involved should know about.
I'm glad to see there's at least one more person who thinks this whole ordeal is at the least strange. Comments on the chapters themselves are mostly complimenting MC for scoring and mentioning how refreshing it is to see an MC that's willing to have sex instead of backing down (which it still is, given this is technically rape).