You claim that where I grew up does not make the situation any more or less moral. I never claimed it did. You presume I assume that my country of origin somehow determines the morality of the world. It's this assumption of yours that makes you presumptuous. How I was raised very well influences what I see as moral or not, but I've never claimed this determines what is moral or not.
Both parties here are adults, in mind at least, yes. You're mentioning I'm citing laws that do not apply here. We don't know which law applies here as age of consent is dependant on the prefecture this took place in and as far as I'm aware, the prefecture this took place in has not been confirmed. What happened could very well be illegal. Assuming this is a prefecture where the MC's age is that of consent, it's still odd that the guy invited him. It'd mean he had to know his exact age, to be sure he wasn't crossing the law here.
The whole reason Age of Consent is a thing is not because the parties involved could be happy with the result of their encounter and people would like to spoil that. It's a thing because when people aren't developed yet they can't make decisions like these. They can be very easily manipulated into doing things that could scar them emotionally, because their minds are underdeveloped. I realize this isn't at risk because MC has a developed mind (assuming it hasn't regressed to that of a teenager since his timetravel), but my issue is that the woman involved didn't know this, yet continued with this. That this guy who's clearly well known about legal situations invited a minor, his girlfriend's brother, to a drinking party regardless.
