Ladies, why does my gf have such a problem with this?

OK. I do the laundry. As a matter of fact, my gf is impressed at the job that I do--especially getting whites clean. So, every now and then I like to take a whiff of her undies while I'm doing laundry. She calls me "sick" for doing this, but my belief is that there's nothing wrong with it, and I should be allowed because I'M doing the laundry! Is there a problem here?