Is it true that sperm cannot permeate the vaginal wall but that it can permeate rectal walls and enter the blood stream from there?

The sperm itself will not change locations through osmosis... but any bacteria or virus that travels with it will more easily end up in your blood stream through rectal walls, just because they tear more easily than vaginal walls. However, vaginal tissues tear as well. HIV travels through the tiniest cracks. It is always good to assume the worst.