To the best of my understanding of making beer, wort must first be properly aerated to provide oxygen for the yeast to multiply when first added to the wort. After all the oxygen is consumed, the yeast switch from being aerobic to anaerobic and begin producing CO2 and alcohol. Having a lot of healthy yeast is important, but the goal is to get them all into an anaerobic state to produce alcohol.
With that in mind, does opening a fermentation bucket and exposing the wort to oxygen cause the yeast to go back into an aerobic state? I'd like to periodically check the gravity of the beer (while being very careful with sanitation) to how much and how quickly my beer is fermenting, but I'm afraid to risk hurting the yeast with early exposure to oxygen.