Marble (and other materials) stain when the minerals and elements in the fluid (or solid) leech into the material.
Sealing doesn't prevent things from staining - it acts as a barrier between the staining substance and the tile and slows down the absorption process.
It's ideal to use cement (not grout) between marble tiles which comes in a variety of colors - or you can add color to it - either way - you must seal your grout/cement when you're done (which is why most people don't seal their tiles before applying - they apply, grout, then seal it all at the same time). . . if your marble needs sealing at all.
At this present moment in time I am making cabinets for the kitchen - just in case you wanted to know what I'm doing when I'm not around.