boys will be boys

Short definition from Wiktionary

Proverb

1. 1
   It is hard, often fruitless, to attempt to curb the natural playfulness and tendency to mischief of most growing boys.
2. 2
   Even grown men usually remain somewhat boyish at heart.
3. 3
   Any potentially objectionable behavior (especially predatory sexual behavior) that the speaker considers to be commonly practiced by many boys and men should be viewed as excusable because it comes naturally to them.