Rapaire,

In English, to spend a penny means to go to the toilet,

however, in this context, to be spent has more to do with one being spent rather than having spent.

After running for the bus my energy was spent.

In that way you can see that someones sexual desire can also be spent.

Or indeed ones seed.

Helpful?

I think you might probably reflect that it isn't an exclusively english thing, but perhaps a use of language common to english speakers world wide.