It's the smell. Every person has their own unique smell. She probably feels closer to him when she is wearing something of his that smells like him.
Also, men's clothes (including shirts) much more comfortable. Way less spandex.
Why does women like to wear their boyfriend's shirts when waking up the next morning after they had sexual relations?
Because WE can't, and she likes teasing you. I mean, you're a big dude (considering you're a guy in this situation) and you try putting on your girlfriend's shirt (who's smaller than you). You might as well be trying to wear kid's clothes.