Software glitch with iMessage & SMS?

Hi, there is an option on the iPhone settings under messages to turn off iMessage (which comes across as a blue message). When you turn off iMessage (iPhone users only) you can still have SMS (cellular texting which comes across as a green message) on. However, the past few months (not sure what update) when you turn off iMessage, the SMS option also disappears and texts that I send out do not go out. I have even deleted old messages to make sure and tested this with several people, and it disappears on their phones as well, so it's not just my phone having an issue. What is the fix here? It seems like a software glitch. What am I missing? I see where you can still do group messaging and send videos etc. But the SMS option disappears in your settings when iMessage is shut off. It is not working properly.