I guarantee you, if I were to put my hand inside you, or penetrate your asshole, I would not hurt you. A partner who really cares will not push you to the point of pain*, unless that's what you want.
* "pain" is somewhat relative. I'm talking about pain that will immediately make you want to stop whatever is causing it. Often, if you tell yourself very mild pain is really discomfort, you won't feel it as pain. And often, mild pain is almostly completely ignored when accompanied by intense pleasure.