E.D.
Both my mom and dad remarried after they were divorced. Each handled their new relationship differently. My mother made sure that the introductions were gradual. She made space for my feelings, talked them through with me, and didn't take my reactions personally. She had developmentally appropriate expectations for me. Most importantly, she didn't push her new partner on me. She allowed me to warm up to them in my own time - asking that I was respectful - but not that I interacted with them as a parent. It took time and was hard at first, but I ended up feeling really happy for her when she was married.
My dad, on the other hand, did the complete opposite. He asked that I accept his partner immediately. I'd grown up spending a lot of time just hanging out with him in the shop and garden, but once they started seeing each other, I wasn't allowed time with him alone anymore. He took it very personally when I pushed her away, and he wanted me to have a grownup reaction to his new marriage, but I was barely a teenager. It was very difficult. I still have a difficult relationship with my step mother.