You can't. While legally, she may be at the age of concent, she has not yet reached the age of majority. Until then, you are still the custodian of her Rights. Once she turns 18, she's all yours.
That's just plain icky even though I'm fairly certain you are bringing this straw-man up for illustrative purposes.
Is she or is she not a consenting adult at age 16 (140 in some states.
I can see why your uncomfortable but this example is precisely why some of us have a problem that the "nuclear" option was chosen to get rid of sodomy laws.