I hate to say this, but could your co-workers assume that because you are the only female that YOU should clean?.
Embarrass them, I say, and a good way of doing it , would be to knock on bathroom door while still occupied and say; " I'm here with my cleaning supplies after you are finished"! They'd probably LET you!
Honestly, Sounds like you work with some morons:(.
This is totally grose, but if you get nowhere, try leaving some used feminine hygiene products in there.