But I still felt really weird for awhile afterwards having had really graphically sexual thoughts about someone who *might* have been way underage.
That quote right there tells you all you need to know. You're fine. Your morals are in the right place. In sex, if something causes you an issue you analyze it and decide what to do about it. That's exactly what you did here, and you finally erred (or not) on the side of caution.
Are fantasies about this girlwoman wrong? No. They're fantasies. Daydreams. Your mind working through desires that you either can't or won't act on in real life. Use the image of the girlwoman, make her 18 in your mind, and be fine. In real life, only sleep with people that make you comfortable. If you have to ask a woman for her license to make sure she's over the age of consent, well, go for it. I've done it to some of the guys I've slept with.
Now... To Maxcock: Consider this. When I was 19, my boyfriend was 17. Was that wrong? The age of consent in the state I was living at the time was 18, and it was pre-Lawrence. I had been dating him since I was 17 and he was 16, about a year and a half. We met on a University campus where I was attending school. At the time I thought he was a student too, not just visiting a friend. Granted, it's not the age difference the OP was facing, but it's still something that by your posts it seems you'd demonize as "pervy."
Also, being gay is not illegal now. As you stated, Lawrence v. Texas ended that. DADT, while still abhorrent, pertains to a voluntary entity in our society. Talk to me again if the draft it reinstated.
Something else to take into account: How many of us here slept with people over the age of 18 when we were teens? I know I did, almost exclusively. The reason, fear. I was afraid of how my peers would react, because I had never met another person who was gay and my age. I didn't until I was 16. By that time I had been sleeping with guys for at least a year, guys I had told I was 18, and who were in their 30s. There was no authority manipulation, and by NO means did having sex as a "child" as it has been put screw me up. Was what I did wrong? Probably, but I didn't care. I was a horny teen male. I fully admit that back then I was the predator. It happens, and society should realize this. There are predators out there, but sometimes they are the ones who are "underage." That's why today I have the rule of carding anyone who is questionable that I may want to hook up with.