What Am I?

It comes in a number

Of sizes and shapes

Required for humans,

But never for apes.

In order to use it

Folks have to be trained

It’s frequented daily

And easily stained.

Off-limits to some,

But oftentimes free

Though some still exist

Where you must pay a fee.

Discomforts an issue

If none can be found

Much more so for women

Whose bellies are round.

And as to the sexes

The men take the lead

For finding replacements
When meeting the need.

A riddle for Napowrimo.net.