Sheepskin do not block STDs.
“The Food and Drug Administration has approved the marketing of male condoms made of polyurethane for people allergic to latex.
“Male condoms now sold in the United States are made either of latex (rubber), polyurethane or natural membrane (called “lambskin,” but actually made of sheep intestine). Scientists found that natural skin condoms are not as effective as latex condoms in reducing the risk of STDs because natural skin condoms have naturally occurring tiny holes or pores that viruses may be able to get through. Only latex condoms labeled for protection against STDs should be used for disease protection, unless one of the partners is allergic to latex. In that case, a polyurethane condom can be used.” (FDA link)