Actually, it's not about offending people, it's about not using language that dehumanizes based on inherent characteristics. I'm a woman, and using gendered language that implies that womanhood is an insult is obviously wrong. Mentally disabled folks were born that way, or something happened to make them that way, and we need to recognize their personhood, not define them by the way their brains function. I'm sure you wouldn't like people to define you by a part of you that you can't change.