But the pressure from the atmosphere applies to both sides of the sphincter, resulting in zero net pressure. Unless the suit actually does press against the outside of the sphincter like it does the rest of the body, I think OP’s concern about the suit squeezing you like a tube of toothpaste is valid.
Maybe the suit only applies a few PSI instead of the full 14.7, which it seems like one’s sphincters would be able to withstand.