The beginning of disrespect might not come solely from your girlfriend, it could come another person. Maybe you were out somewhere and somebody else disrespected you in front of your girlfriend, that will get the gears going through her mind. If you appeared weak, scared, or frightened during that encounter she will look at you as not a very strong man. She won't feel like you can protect her and she won't feel safe with you around. You will feel emasculated and won't have much confidence. If she sees another person who is able to disrespect you, then she will be able to do it as well. This is another point where the trouble will start if something like this happens.
Similar situations happened to people I knew, and right after that, the chick started dissing the guy. She saw him in a negative fashion and started looking for a better man. He lost control of the relationship while he spent his time trying to please her doing what she wanted. As usual, she got tired of him and left him in a cloud of dust as she headed off into the sunset with her new man.
Some fat women who are "large and in charge" like to bully their men.and are completely happy disrespecting him. They know that they won't be able to get away with this with another man, so they abuse their little beta man. But after a while, even they get tired of the dominant role and look for something better.
So, don't ever be in a situation where you can be disrespected or allow the woman to fully disrespect you. Sooner or later you won't have a relationship anymore.