I have, many times. I like feeling my hole filled up. And knowing I'm essentially having butt sex while going about my normal day is pretty hot.
Since you "Don't know how", maybe you should avoid being so judgy and calling it "Nasty!"
If it's not for you - and it's 100% fine if not - then just say "I wouldn't do it" or "that's not my cup of tea" and leave it at that.