Not as much as you would think. How would YOU like it someone wanted to stick their dick into you on command. Believe or not it CAN be uncomfortable for a woman. Sometimes she just isn't wet enough down there or just doesn't feel like having a dick in her.
Just because >>1 can't keep his dick from getting hard all the time is no reason for him to whine. Sex can be good, sex can be great but males don't always realize how much of an imposition it really can be.