Gosh, most everyone is concerned about STI/STD's -- and they should be. You're right, there's no such thing as 100% safe sex, though of course some infections are more infectious than others (and some are way more serious than others).
Your best protection in the long run is probably studying up on the various infections and being aware of the risks. Wikipedia is your friend. Also regular testing is a good idea, though it too tends to have its limits. I don't think they usually test for *everything.*
I'm kind of "lucky" in that my V has a closed arrangement: No sex outside the three of us, unless a fourth person entered our group which would be a long involved process and we'd need to get to the point where all four of us *really* trusted other.
But even then, you see, we're relying on our trust for each other as our "protection." Technically, one of my companions could go out for a night of wild sex with seedy strangers and I wouldn't know it. I just feel, strongly, that the chances of that would be slimmer than the chances of Yellowstone exploding (destroying civilization as we know it). Granted we couldn't stop Yellowstone from blowing up if it wanted to, whereas we could use condoms and/or get ourselves tested regularly. But since the odds seem so favorable, I guess I'm just willing to take that risk.
Cuz as I said, there's *always* *some* risk.
