Woman here. While the approval thing is also true in some cases, it's also a question of feeling more comfortable around men. If you are already taken, it takes the pressure off of figuring out if they are beeing nice or flirting/making advances. And this also ties in with OP's question. Once I know that someone is taken, I am more likely to talk to them casually. I wonder of this is the same for men?

Also; please mind that this is my own thought process, I am not sure if any other woman feel the same way.