The proverb is real, although spoken differently.
In episode 5 of Kenichi: The Mightiest Disciple, Miu said the following proverb,
男子門口を出れば七人の敵あり (danshi kadoguchi wo dereba shichinin no teki ari)
A young man, if he steps out of the gateway, there are seven enemies (lit.)
The more known proverbs are:
- 男は敷居を跨げば七人の敵あり (otoko wa shiki'i wo matageba shichinin no teki ari)
- 男子家を出ずれば七人の敵あり (danshi ie wo izureba shichinin no teki ari)
- 男子門を出ずれば七人の敵あり (danshi mon wo izureba shichinin no teki ari)
which its interpretation is more about
A man has many competitors, if man comes out to society.
This can refer to education, career, and similar situations (basically: people are competing with each other outside; always be on your guard and do not open a chance to be used by others)
One of the earliest references is on Kunio Kishida's essay Nihonjin to wa? (1951) (Japanese), but no specific origin is known.
Since a long time ago, there were sayings like "if you see a man, think of him as a robber", "if a young man goes outside, there are seven enemies", anyway, it is told that people should not be familiar with each other naturally, rather, they are unforgivable to each other, and if they are not careful, an opportunity can be taken.