The first few instances would surely be challenging, the lack of lube, the difference in sensation as compared to penetrating vaginas, and the possible accidents every now and then.
But once you hit the right spot, it should be smooth sailing from here on out.
Com is definitely the website for you! There are flat asses, perky asses, round asses, wide asses, you name it! And whatever they can fit in their butts! It would surely hurt so much, with the possibility of giving her breast cancer as well.