Correct me if I'm wrong but as far as I checked, what it means being underage in Japan, the 20 years of age makes you actually eligible i.e. to drink alcohol or smoke cigarettes (not that I would recommend it)
When it comes to the age of consent ... it's actually 18 years so legally Jun is 'ok' ... I'm not referring of course to his public image or reputation which must have been hurt.
Btw, it's little funny that a young person i.e. of the age of 18 is not ok to have a beer, but still ok to have sex ... and this is the situation in many Countries I guess, not only Japan. As I followed the links, I was shocked to learn that there are some countries, where the 'age of consent' is ... well ... 12 (no comments here). Japan's 18 is actually one of the highest one, Bahrain is 21 and an average for other countries is about ~16.
The issue here is more than age of consent. The point of contention here is whether he's committed infidelity, which is punishable in Japan where her wife could divorce him on that ground and claim alimony. This is why he and his acquaintance keep stressing he didn't cross the line(physical relationship).
Cheating, affair and infidelity
In the first place, the difference between “cheating” and “affair” is easily confused, but strictly speaking, the meaning is different.そもそも「浮気」と「不倫」の違いは混同されやすいものですが、厳密に言うと意味が異なります。
In general, cheating often refers to a general relationship with a person other than your spouse who is in love or is suspected.
It is also used between lovers who are not married.
On the other hand, infidelity is often used in the sense of having a physical relationship with the opposite sex other than the spouse, that is, having sex. It is also a term used between married couples.
However, neither is actually a legal concept, and the affair of a spouse who is legally married is called “unfaithfulness”.
The reason why it is necessary to distinguish between cheating and infidelity (unfaithfulness) is that legal claims for “claim for alimony” and “claim for divorce” are not allowed unless they are infidelity (unfaithfulness).