Anal is hard not just because you have a thick dick fucking you hard but because you’re fighting against natural instincts and forcing your body to do something not natural. When anything is inside the rectum, instinct is to push it out especially when it’s near the opening/hole (most sensitive.) The sensation to push is increased when there’s some large is inside your recum because all the pressure points are being hit. It’s not natural for anything to go the opposite way. So having a large penis enter your recum and hitting those pressure points makes it hard to not push out the dick. Eventually the bottom will get tired contracting their muscles and relax making it easier for the top to dig in their butt deeper. The ass meat on the inside will be tenderized and the hole will be softened.