Are you following along so far?
Now, next is why erections are a no-go. When a boy gets erect, it's easy to tell just through his pants, isn't it?
In other words, when a boy's lust swells, it's plain for all to see.
By allowing his dick to grow so thick and hard, it's only obvious he'll end up putting any surrounding females into heat too.
After all, "I want to knock up a bitch" is what a boy's erection means... you see?
Convenient form of common sense imprinted