i'm straight, but i've still shoved a few things up my asshole. a toothbrush, a bullet vibrator, a butt plug, and a dildo vibrator. the bullet was the best, used it a couple times when masturbating, felt pretty good.
the butt plug was the worst. too big for me since i had no experience, basically made me fall to the floor and crawl when i got it all the way in. felt like it was brushing against my guts and organs and that scared me senseless.
the dildo hurt too and i didn't use it again, it was 7.5x5.5 inches. i was bleeding a bit after using it. gave it right back to my girlfriend. the butt plug must have been fatter than that but i never measured.
i've tried shoving things into my urethra but i'm a total beginner so it hurt and i stopped. i guess you have to be an expert for that kind of thing.