Being Proship means that you believe that fiction does not affect reality, so anyone can ship whatever they want. People use it as a way to excuse shipping illegal stuff like incest and pedophilia, along with generally abusive or toxic relationships.

The idea is 100% false; fiction can very much have an effect on reality, and seeing support for these things in fiction can desensitize someone to them irl, or make them believe that it’s normal or ok. It’s an idea that has become horribly common, and does a lot of damage to people. If you want stories of the damage proshipping has caused, they are relatively easy to find.