Steve, it is weird. And I'll give you my opinion. You're safe. You're easy. You give her what she needs. Security. But why buy the cow when you can get the milk for free. Ever heard that term before? She wants the benefits of a boyfriend, without the commitment of a boyfriend. She probably does like you a lot. She probably likes him a lot. She probably doesn't know what she wants. Or maybe she does, and what she wants is NOT to be in a committed relationship. Sooo... I think you need to ask yourself, "Am I happy the way things are?" and if you're not, then you need to be prepared to walk away. Because the next question to her is "I'm tired of being your half-boyfriend. It's in or out, but none of this half stuff any more. You decide and what ever you decide is perfectly ok with me. But you need to decide because what I need is a relationship and and not a makin' out friendship." And THEN you have to be prepared to accept her answer whatever it is.
Don't settle, Steve. Life is way too short. If you think this gal is great, I PROMISE there are a ton of women out there that would LOVE to be with you. I know that might sound a little callous against her, but it's not. Simple truth. If she don't want you, I promise someone else will.
Good Luck! Brian