I am all for using some kind of barrier that works effectively, even though it is rare you can get STD with oral, I am definitely not apposed to it if that's what a guy prefers. You can also make your own barrier that is comfortable and effective in case you need one on the fly  
   
 What is an oral barrier?  
   
 Oral barriers, usually called dams, are barriers that reduce the risk of contracting sexually transmitted infections when participating in oral sex. An oral barrier is a thin sheet of rubber latex or polyurethane in a rectangular shape that measures approximately 10" x 6".  
 How does an oral barrier work?  
   
 The oral barrier works by preventing skin-to-skin contact or contact between body fluids when participating in oral sex and can be used when performing oral/vaginal or oral/anal sex.  
 How effective is it?  
   
 When used correctly each and every time a person has oral sex, it can significantly reduce the risk of getting an STI. It cannot take the risk away completely.  
 How to use an oral barrier  
   
     For oral/vaginal sex, lay the dam flat and cover both the vaginal opening and the clitoris. Hold the dam in place.  
     For oral/anal sex, lay the dam flat and cover the anus. Hold the dam in place.  
     To hold the dam in place for hands-free pleasure, purchase a holder from a specialty shop or use a garter belt.  
     Do not stretch the dam.  
     If lubricant is desired, use only water-based lubricants on vaginal or anal areas before putting the dam in place.Take care not to get lubricant on the fingers as it might make it difficult to hold the dam in place.  
     Each dam should be used only once.  
     Always use a new dam if you switch from oral/anal to oral/vaginal sex to reduce the risk of infection from anal germs.  
   
 Advantages  
   
     Helps reduce the risk of catching sexually transmitted infections  
     Available over the counter without prescription  
   
 Disadvantages  
   
     Some people may be allergic to the rubber latex  
     Reduces sensitivity  
   
 What if I don't have a purchased oral barrier?  
   
 You can easily make your own barrier out of a condom. Use a condom that isn't lubricated with spermicide since you'll be putting your mouth on it. They're simple to make.  
   
 First unroll the condom, then:  
   
     Cut off the tip.  
     Cut off the base.  
     Now cut down one side.  
     You now have a square latex barrier.  
   
 You can also cut up a latex glove (not a kitchen glove) to make an oral barrier with a bonus. Get the kind without talc inside, since again, you're going to be putting your mouth on it.  
   
     Cut off the fingers.  
     Cut off the base.  
     Now cut along the thumb side of the palm (Go ahead and leave the thumb on).  
     You now have an oral barrier with a bonus thumb-handle. You also now have 4 bonus finger cots, which are especially recommended if you have cuts, bitten nails, or torn cuticles on your fingers.  
   
 You can also use, if necessary, kitchen plastic wrap as a barrier during oral sex. The good part is that it's big and it is transparent. This lets you see what you are doing. The bad part is that sometimes it sticks to itself, although you can also get it to stick to the other person, which keeps it in place.