I just read through this poem several times and...I think I agree with you. While literature brings us the wisdom of the world, we must recall that it is human wisdom and prone to error. She seems to be saying, "I want you to be in love with the idea of being in love," which (as you say) leaves her adorer's love open to anyone.
She clearly wants his love to transcend anything that is liable to change--not just her looks, but the things she says--but logically, her suggestion to "love for love's sake only" doesn't work. It ultimately has no hook upon which to hang.
Then again, some people can have faith and believe for the sake of believing, so why do we find it so improbable that a person can love for the sake of loving? And what are we thinking, anyway, trying to be logical about love?