I would ask her what "natural" means to her. Perhaps she could use a different word that might shed more light on the situation. Maybe your idea of "natural" and hers are different.
I agree that men and women see sex differently sometimes. Men have sex with women to show and feel their love, whereas women have sex with men when they have shown their love and feel it before hand. If that makes sense... at least this is what I notice. For example, Mono approaches me for sex because he loves me and wants to show me that. As does PN. I love them and have sex with them because of that love, not to express that love... I have sex with Derby because of love and appreciation for each other, not because we have to show each other.
I would suggest, like others have, that she is not into you that way. she has no reason to change anything because she likes what you have, but doesn't want to jeopardize her strong friend-like loving feelings by having sex.
I have a Non sexual boyfriend that I am the same way with. I don't see anything wrong with it because we understand each other and what the relationship is. It works because we both agree... what I see here is you not agreeing. So, the question is, what will you do about it? Will you just carry on, ask for more, or break it off... that is up to you.
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