What's confusing about it? I'm honestly confused by your confusion. When I want to message someone, I use Messages. Whether they have an iPhone or not is irrelevant, because that will be handled in the background.
Then disable the fallback... It's a very obvious and simple setting.
When initiating a new conversation, the number you add to it appears blue if they can receive iMessages, green if not.
If for some reason iMessages can't be delivered to the receiving party at the moment. Text messages are used as a fallback for them. If you disable the fallback mode. It won't be.
The whole thing is kinda intuitive, and works really smooth IMHO.