My worry is that my anus will permanently stretch out, and never return to its previous tightness; this is a great concern for me because, as a virgin, I'd like to be tight for the first time I receive anal from a real guy, and not all loose. I'd heard there were excercises to help keep the anal muscles tight, but I'm worried those might not work. Start by inserting larger and larger objects into the anus to stretch the anus. Kind of like inserting one finger, then adding another, then another until you have 3 or 4 fingers in or the whole. The Rectal Sphincter/Anal Sphincter is an approximate 1 inch ring shaped muscle that surrounds the external opening of the rectum.
The rectum is a collection of muscles and skin and mucous membranes. Everything here is stretchy. Like, really stretchy. Normally, the external anal sphincter closes water-tight, but opens up to pass waste leaving the body sometimes to sizes alarming to the person so passing. As others have noted, there are fetishists who practice the insertion of large objects into the rectum, with a mind boggling array of shapes and sizes and varying degrees of firmness. A brief period of time with the Internet search how far can anal sphincter stretch of your choice and some creative phrases will yield photos of much of this sort of thing.