Carry on then. With all the SU hate threads that are spawning everywhere, one starts to get ticked off from the bad ones as well.
Anyway, here's a proper explanation.
The body of a gem is a hologram created by the stone, and that's a fact. But if two or more gems are able to think in sync, they can join forces to make one single, bigger hologram that fuses the gems' powers and personalities. (In Steven's case, the hologram is replaced by some kind of holographic energy that allows him to reshape even his organic matter, I assume)
Now, fusion can be used by the show's writers as a metaphore for relationships, either sexual, amorous, familiar and whatnot. That's because to work correctly, a fusion requires the same kind of harmony and lack of codependency that is required from a human relationship: it's literally the ultimate form of relationship, one where you have to be one with the person you relate with. However, and that's where the cleverness comes in, fusion is not necessarily a metaphore in every appearance. It can be a battle technique, it can be a way to join forces in a way simple teamwork will never allow you, etc. Fusion is a tool with multiple uses, both for the show's characters and for the show's writers. I don't know if I made myself clear.