I haven't used a condom in many years; can't even remember the last time. The reason I don't have fear, other than the fact that I gave up hooking up years ago, is because I realized long ago just how easy contracting an STI can be: I got syphilis from receiving a blowjob. I'd say the number of guys who want oral sex, whether giving or receiving, to involve condoms is close to zero.
That aside, if I understand correctly, if someone has an STI but no symptoms (i.e., they don't have blisters or discharge), the odds of spreading it are lower. Obviously, if the guy who gave me syphilis had a chancre in his throat, I wouldn't have known.
All that said, if a guy wanted to wear a condom to fuck me, I'd be okay with it, even if I do love taking loads. Unless a guy is a total top and absolutely cannot get pleasure while wearing a condom, I think it's silly for it to be a dealbreaker.