If both partners enjoy a blowjob with a condom on then go for it. If they are doing it for disease protection knowing the man has something then good on their behalf. Usually the stomach acid will kill germs the cum may have. However, the longer cum lasts on your tongue and in your mouth the better chance for you to catch a STI. So suck it and swallow it fast. Don't play with it in your mouth.
Me personally I will suck a cock without a condom each time. I want to feel the heat of the cock on my lips and tongue. I want to taste the wet flesh from the spit and precum juices. The most Important part, I want to taste and swallow every drop of cum a man will shoot for me.
I ♡ Cum!!!