Look... I have a 19yo son. Just around the time he turned 18, he was dating a 16yo girl who was WILD for him. Loved him - Truly, Madly, Deeply. Her Dad said no. Actually, not *just* no. Hell no. Hell to the bells no. Hell to the "put your mother on the phone, son - I wanna talk to her" no. It was made very clear that if my boy didn't back off and leave her alone, the authorities would be involved. So I had a sit-down with him and told him that he continued with this girl at his own peril. He was smart. He looked at her, and he looked at his future. And he walked away. Now, she was 16 - perfectly legal to be sexually involved in our state. But not to be in any way involved without her parents' consent.
If you REALLY love her, you will wait on the sidelines until she's 18. But...