This post was inspired by a brilliant comparison Sarah made. I've embellished it a bit, but the central insight is hers. As usual, I can't tell where her part leaves off and mine begins.
I'm a hunter. My granddad was a famous hunting guide and my dad runs a hunting club. So I've grown up around guns. One practice I learned early on was that you never point a gun in the direction of a human being (who is not attacking an innocent person): you "treat every gun as if it were loaded". I am now unable not to abide by this. I'm actually unfree to point a known loaded gun at someone, it's so contrary to my nature that I can't make myself do it. This is so because of the good training I had on the topic (Aristotle would be proud, the chief aim of early child-rearing is to *train* children to have the right *habits* so that later in life they don't have to struggle to form them).
But another reason I can't point a gun at someone even if I know it's unloaded is that even though I know it's unloaded because I just checked it, there is always that tiny chance that somehow a bullet got stuck in there which I didn't see. That chance is very tiny indeed. However, the consequences of shooting an innocent person are bigger than I can calculate. So multiply that tiny chance by those huge consequences and you still get a huge reason not to point the gun in their direction.
HOW MUCH MORE then should we so act with a gun we know is loaded but we know has the safety on. Safety's work by physically blocking the trigger from pulling back far enough to release the firing pin into the primer of the cartridge. It's very unlikely that one would fail, but it is not unheard of and I have thought one was on when it wasn't and I have accidentally pressed the safety off. It is rare, but it is a reality.
Now about sex. A condom is a safety on a loaded gun. However, the chance of failure is greater and so are the consequences. Killing someone isn't *nearly* as momentous as *creating* someone. No life is ever truly ended and earthly life is of strictly finite value anyway. But when you create a life, that is an act of infinity. That soul never ends, ever. And the human life of that person might be really messed up as a result of thinking of themselves as an "accident" which caused the misery of their parents who now had to get married. This is a common tale.
The act of sexual intercourse is intimately connected--if you will--with procreation. That's Bio 101. No act seeks such union with another. It carries with it an implicit commitment commensurate with that. And since the fruit of successful procreation is nothing less than a human life, sexual intercourse carries with it, inherently--whether the parties are thinking about it or not--the promise of a lifetime commitment. Thus sexual intercourse not backed up by such a willingness is a sort of lie. It says something untrue.
So, to make the analogy explicit: to have sexual intercourse with someone to whom you have not made a lifetime commitment is like pointing a gun at someone and pulling the trigger in the *hope* that it is unloaded or that the safety will work. It is not nice and it is not safe.