incomprehensible I don't think it works like that. HIV for example has a transmission rate approx 1 in 1230 times. So it is unlikely that HIV is transmitted from one occasion, but you just don't know if you're going to be lucky or unlucky on any given occasion. There are also a lot of other factors that affect transmission rates.
As for other things, I have no idea but generally some things are very infectious some are harder to pick up. So if you want to go without condoms and stay safe, best to both get tested.
I looked this up (again) and I'm horrifed to see that the fellatio rate is as high as unprotected vaginal sex page down to table