Anyone is more than welcome to bring their infant to Disney, just as you are more than welcome to bring your 63 year old knees
That being said, I go to WDW about 3-4 times a month with my DH, and 2 year old DS, and the 8 month pregnant belly I'm carrying around. Often, no one offers me a seat on a bus or monorail and when they do, I usually decline. They waited longer for that seat than I did and having a child with me or being pregnant certainly doesn't make me disabled. I know it's easier for a woman with a baby to sit down and MUCH safer so I always offer my seat.. I do the same with the elderly as well. Just yesterday I gave my seat up on the monorail to an older woman who refused several times to have me stand up, but eventually I convinced her.
The annoying part was the 5 or 6 teenage boys sitting comfortably in their seat with no concern for the older woman who obviously really needed a seat.
If we are on a full bus, my DH will always stand because he's able. It's just common courtesy and the way we were raised. It could be a woman without a baby and she would be offered his seat. I've done my share of standing on a bus with my baby in my arms, not being offered a seat, but it doesn't bother me. I don't expect a seat, but I will give mine up because it's the polite thing to do.