Step parenting

Very upset about the way I am viewed as an authority figure in my household. I feel as if I'm good enough to pick up after, run errands for, spend money doing fun things, and take care of my step child...BUT not good enough to make decisions when it comes to discipline. The other morning after a rule wasn't being followed I calmly spoke to the child and asked why they were misbehaving, then asked my SO if he was going to discuss it with them. I was immediately accused of being too strict, and basically cruel to the child. I feel so disrespected...and furthermore sad that my SO doesn't have my back when it comes to parenting, especially since we have a little one on the way. I don't know what he expects when I WILL have a say so in our child's discipline, because I treat his child as if they are my own. What can be done?