Tell her you won't do that.

"No." you say. "I'm sorry but I'm not doing that."

The conversation ends quickly after that. Sarah is not interested in talking to you.

You spend most of the evening wondering why she would even want to treat you like a baby. Getting nowhere, you turn on your computer and search for "Why does my girlfriend wants to treat me like a baby?"

Most of the results aren't about literally being treated like a baby. They are guys complaining that they feel their girlfriends are looking down on them. However, hidden amongst those, you find that there are many adults who like to dress up in diapers and babyish outfits and some even have partners who treat them like babies.

That night you have a strange dream. You are sitting in a giant cot when Sarah peers over the railing at you.

"Good morning little one." She smiles down at you. "Does my baby need his diaper changed?"

You start to protest that you aren't a baby and you don't wear diapers but look down and discover that you are wearing an extremely thick disposable diaper with cartoon characters printed on it.

Sarah doesn't wait for an answer. She reaches down and somehow picks you up. Holding you against her chest, she pulls out the back of your diaper. "No poopies but I think you might be wet."

She lays you down on a changing table and leaves you there for a moment while she gathers the necessary supplies.