How to politely get my girlfriend to wear underwear?

My girlfriend doesn't wear underwear. Just leggings and due to this there is hygiene issues down there. She says it's uncomfortable and says she will never do it. She doesn't think the hygiene issue is a big deal or that it comes from the lack of underwear. Even though to me it is when i have to smell her anytime she isnt wearing leggings. Or when she want intimacy but I just can't due to it.

Is this normal? I've not been with many girls and my ex wife before this never smelled. So I'm confused what it is. And if it's going to hurt her long term down there not wearing underwear