No pretreatment needed when not printing any white ink.
The correct term is "necessary"... Pretreatment is not "necessary". You can print without pre-treatment, but the print will not be as sharp as when using pretreatment.
NO CMYK colors will stick to pretreatment by themselves.
Nothing sticks to the pretreatment as it is water soluble and it is meant to be washed away. This is also why using too much pretreatment will cause poor washability.
The why:
Pretreatment is NOT a binder. What it does is gelling the ink upon contact, preventing bleeding into other colors, or sinking into the fabric.
Possible Washability problems caused by pretreatment:
1. Using too much or the wrong mix ratio when printing white/light garments.
2. Using too much of the correct mix ratio on any garment.
Don't take my word for it... Try it.
By the way, the maximum ratio for white/light garments is 30% pretreatment in 70% water, and 50/50 for dark.