No, sex doesn't make you stupid. But it relaxes the mind, clears away worries such as kids and bills. For a while, you stop thinking about anything but how great you are feeling, and how much in love you are. After a cooling-off and calming-down period, your knowledge and brain cells are all still intact. And maybe you are a little smarter, like if you tried something new in the sex and now you have that knowledge and experience on top of everything you already knew.
Does love and great sex make you ACT stupid? Well, not stupid so much as goofy in love and lust. You do kooky things you wouldn't have done before you met that partner. But what's wrong with THAT?