Hey, I agree with him, some of this stuff is scientifically questionable. The vagina is definitely *not* impervious to viruses, as the number of women infected with HIV through heterosexual intercourse attests. I'm also sceptical about the claim that sperm promiscuously fuse with any old cell, as that would pose an equal cancer risk to a woman's reproductive tract if it were true. In fact, the cell membrane of sperm contain markers that only will match with markers displayed on the membrane of an egg cell, so fusion with somatic cells should be impossible.
Anal sex does carry a higher risk of HIV infection than vaginal sex, but that has been attributed to tearing of the protective barrier of the mucous membranes.
The major reason IS due to the immune suppressive properties in sperm.