And yes, you can force it !
Go into settings->messages and turn off "send as SMS if iMessage is not available"
You will need to answer your android friends sms' with WhatsApp....
That setting affects your messages to other iMessage users. It doesn't affect your SMS messages to non iMessage devices.
If you have the setting turned on, iMessages will fail over to SMS if iMessage isn't available for some reason. With it off, iMessages don't fail over if iMessage is unavailable for any reason (so they typically stay in "sending" state from my experience).
I have this setting turned off due to my provider now charging for SMS to the USA (I'm in Canada). I have family in the USA and occasionally iMessages would fail over to SMS but now that would cost me money. So having it turned off saves me unexpected costs.
I have several Windows Phone, Android, and Blackberry friends and this setting does not affect my messages to them. All still send and receive as SMS without issue.