Important Warning! - Lambskin condoms have been found not to be effective in preventing HIV or STD transmission and therefore should not be used for that purpose.
Condoms - HIV Prevention - Condoms to Prevent HIV
Lambskin condoms are porous -- very finely porous, but porous nonetheless. The pores are small enough to prevent sperm from passing, but the HIV and other STD viruses are much smaller and can pass through.