Just to add some more info to this post…
You can't get a lot of STDs from oral sex/cum, but there are two which people can get: herpes and hepatitis; herpes only if you have contact with skin lesions, but hepatitis can be passed through saliva, which can pass the skin of the penis.
Hepatitis is a "double way" infection (as I usually say), it can pass from the one who sucks, or from the one who gets sucked.
About HIV, as someone said, it is really hard to pass with oral sex. Just to give an idea, I think that there are only 4 clearly documented cases in Europe, that it happened.
But, as an health professional, I think hepatitis is worse than HIV, so choose well the people with whom you have fun folks :sucky: