Well, two extremes exist:
1. Having sex with people whom you know perfectly well. No condoms are necessary, unless these people have STDs and you still for some bizarre reasons want to do them.
2. Having sex with random, crack-addled hookers. Obviously this is unsafe even with a condom.
But stuff exists between these extremes. There are degrees of sensibility between 'random one-night stand' and 'marriage'.
A lawyer is less likely to have STDs than a hooker, for example. Sex with a lawyer whom you've known for a month is less likely to give you something than sex with a random hooker.
Consider it as a math exercise:
Suppose a person you know has, eh, a 50% chance of having something. A condom would reduce your chance of contracting it to, say, 5% (actually more, but I'm just illustrating a point here). Now that's unacceptable for many people (say, me), but it's not the same thing as unprotected sex.
What there's a 10% chance the person you're doing has it? (Of course, the more horrible stuff is far less prevalent. 0.6% of Americans and 0.1% of Israelis have AIDS. Conversely 1 in 4 Americans have herpes).
Now, I'm not saying condoms allow you to do whatever the hell you want with sex and pick up random Swazilandi illegal immigrants and have orgies with them.
I'm just saying condoms are safety equipment. There's stuff you'd try with a helmet on that you'd never do naked, right?
