Is it right for my step-father to spank me at 20 years of age?

Question:

I am 20 years old and my mother recently remarried a man who believes in spanking. Well, the other night I stayed out overnight at a friends house and didn't come home (my mother knew about this) and when I came home the following morning he was waiting at the front door for me. He told me to go up to my room and he would be there to speak with me. When he came to my room he told me that my mother said I was coming home that night and when I didn't, I was showing disrespect, and now I was to be punished for it. He wouldn't let me call my mom to ask her if she knew that I wasn't coming home; instead, he spanked me. He left the room for about 20 minutes and returned and said he called my mother at work and she said that I never told her that I wasn't coming home, so he accused me of lying and turned me back over his knee and spanked me again. Is this acceptable, since he is not my father, and isn't 20 a little old for a spanking? I never misbehave and now I am scared to do anything in fear that I will be spanked again.