So you want an opinion on the law without any moral considerations...you are asking for a PERSON's opinion but the person is supposed to keep his/her morals out of it.
I'm not an attorney but I'd say the laws of our country come, in one way or another, from the collective MORALS of our society. Therefore, asking for "the law" without "moral considerations" is farily ridiculous.
Aside from that, the plain fact is, you are 18 considering having a sexual relationship with a 16 year old...while I'm sure you feel VERY grown up and very much in love (I know I did when I was 18), the fact is, those who have been around a while longer than you understand how much growing up you have to do yet not to mention how unlikely it is that you'll still be with this girl 20 or 30 years from now. So, when they give advice, they are probably going to do more than just offer cold, hard law but they are going to speak from their experience as well.
Aside from that, while you didn't ask for moral advice, YOU SHOULD - most
important decisions are, ultimately, moral decisions.