Yes it is about brain power. For women it's emotional too. Men more physical. Men seem to be able to turn it on like a switch. Women need a little more prep work. Not just, a grab here and there and "Why aren't you in the mood?" When a man comes home stressed he wants to release his tension. When I am stressed (usually from little children climbing all over me all day long. I'm not a jungle gym.) I want the closeness, but I don't want to feel like I'm being climbed on again.
Women are crock pots.
Men are microwaves.