The vast majority of what goes into any vagina that ends in a uterus (it's not just trans women who don't have one of those, since some cis women have had hysterectomies as well) doesn't travel *into* the uterus either. (And never the ovaries!) The cervix is usually pretty tightly closed, and when it's not, it's still not permitting a great deal of material to get through. In other words, cis women have to deal with ejaculation that slowly flows back down over the course of a day or two, too. And yeah, it's not always that appealing a process, but it all does eventually get cleaned out.
The people who say that vaginas are generally - barring infections and other abnormal situations - entirely self-cleaning are correct. "Routine" douching with anything that can alter the vaginal flora is definitely a bad idea; in fact, Brassard's material is very firm about stressing this point. He recommends douching for two months immediately after GRS to keep the surgical sites as clean as possible, but once things have healed up, he says to stop and let nature take over.
