Unfortunately. AD(H)D does impact brain development. People have brains that are and wothout care remain underdeveloped in a few regions, as well as the connections between these regions. So the use of retard is correct.
I can't blame people for not wasting energy on a child that needs a lot more structure, support and training.
As for relationships. They do ask things that people on the spectrum struggle with. From being able to put oneself in the other's shoes, to being aware of how they feel themselves about a situation. Struggling with emotional regulation can also cause problems, while not being aware that what they said or did wasn't okay.
Take my dad for example. He destroyed family ties between my mother and his side of the family. He compartimenalized me as part of "mom's" side. Reducing what little family connection I had. All because of hypersensitivity and not wanting to be around "too many people" once a year for christmas. So to this day he has two christmas parties. With his family, and us. Even though everyone on "his side" is on the spectrum and would have understood if he communicated about getting overstimulated. No one bothered to ask why, or what my sister and I thought of it. As after all, they struggle with theory of mind.
With such carelessness or lack of awareness, I cannot blame someone for not thinking and just running away. If someone is seemingly shortsighted, unreliable or uncaring, why stay with them? Or starting a relationship with them with no certainty things will improve? It's wrong to believe you can change or fix someone, it is foolish to try when you don't.