Hi, qualified veterinary nurse here, you can't gross me out. 
I've seen some pretty incredible things come out of cats (and dogs), and I've also seen some things get stuck inside and have to be surgically removed. I'm pretty sure that it must have gone through him (unless he somehow managed to sit on something and get it in that way??), although how it looked clean after coming out of his butt, I can't imagine. Is it out completely now?
Please keep a close eye on him. Check his colour (lift his lip and look at his gums to make sure they are a nice healthy pink colour). Make sure he is eating and pooping ok, even if you have to keep him inside or follow him around outside for a bit.
If he shows any sign of lethargy, not wanting to eat, not being able to go to the toilet, or straining while trying, please get him to a vet as there may be something still stuck inside.
Let us know how he gets on. 
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