"This means that the initial metabolism will be aerobic. Aerobic metabolism of sugar yields no alcohol, but still reduces the gravity."
Well, actually this isn't true in virtually all fermentation situations involving Saccharomyces yeasts. S. cerevisiae is what is known as Crabtree-positive, i.e. it experiences the Crabtree effect.
What this means is that, at glucose levels above somewhere around 0.1-0.2%, purely oxidative physiology (respiration) is overridden and the yeast produces ethanol and carbon dioxide regardless of the presence of oxygen. For comparison, you could expect to find glucose levels of roughly 1% in any wort you might try to ferment. By the time this glucose is utilized and this repression of respiration is lifted, all oxygen will long since have been taken up, ensuring anaerobic conditions for the rest of the fermentation (hopefully).
However, oxygenation can have an effect on alcohol production that is completely unrelated to this. Higher levels of initial oxygen may lead to over-production of yeast biomass (high levels of yeast growth). In this situation energy derived from sugar is diverted away from the alcohol-producing metabolic pathways towards biosynthetic metabolism. So sugar that would otherwise have been utilized for ethanol production instead goes to making new yeast. In some situations this can reduce the effciency of a fermentation (from an ethanol yield standpoint). In reality it's not likely to be any kind of issue for homebrewers, though, and so isn't much to worry about. It is the kind of thing that very big breweries may worry about though, as small changes in efficiency can lead to very large costs.