First, the liquid that a guy ejaculates is called semen. Sperm are the tiny reproductive cells that are in semen. Even though there are about 300,000,000 of them in each ejaculation you can't see them. (Just want evryone to learn the proper terms).
As long as your partner is disease-free there's nothing in semen that will hurt you. Remember though, STDs including HIV can be transmitted by unprotected oral sex.
Not if your comfortable with it. Plus it probably makes your partners happy. lol