I like my girlfriend for her armpits
Why do you like them?
It's an awkward way to put it but I'm into armpit hair.
That's an uncommon preference, but there's nothing wrong with it. If your girlfriend also likes it then I'm sure you're very happy together. If she doesn't then it might cause problems, though.
It's fine, I love it.
Just because you're okay with it doesn't mean they are. You should just be upfront and honest with them. They might enjoy that you're into their armpit hair and be more open to your fetish.