From what you say here this sounds like a boundaries problem. In your teen years, when so much else is going on and you are going through so many changes, there are enormous pressures on you and also on your parents. I remember that from when I was your age; my parents were rather traditional and it felt like they were driving me crazy sometimes. Now I am a father with two kids of my own, so I have seen it from this side now as well. It isn't easy, I can tell you that for sure!
I had an experience similar to your Thanksgiving problem when I was 19. We had a break from my university, and I decided that instead of going home I would go stay with a friend in another city so we could go see the Who in concert. My mother in particular was incensed at this, and she had a lot of questions that I considered really invasive and insulting, as if I was still 14!
What really irritated me was that this was just part of an ongoing pattern. I got hassled about drinking, drugs, girls, changing my field of study to history (my mother said that historians become atheists!), where I was going, the music I liked, the way I dressed (yep, a hippy), responsibilities around the house, everything! Like you, I felt like I was being cornered and disrespected.
As a father who has gone through this with a teenaged son, I can tell you that the problem of boundaries is a rough one. I tried to give my son the room he needed to grow and develop and make decisions on his own, while still being there to intervene when things looked really bad or when he needed my support. Everything seemed to be changing on a daily or even hourly basis, and I felt like I was just guessing a lot of the time. And when I got it wrong or there was a conflict of opinion, well...I am sure you can guess the rest!
Good parents are always worrying about their children, and I'm afraid you being 18 doesn't change that a lot. It just takes time to let go I suppose. My guess is that your mother was looking forward to you being home for Thanksgiving, so she was hurt that you made other plans (like mine was when she found that I was "choosing" the Who over her). She just had not anticipated that this might happen.
You ask did your mother mean to ask is your friend gay. I have no idea, of course, since I don't know your mother. But my guess (and that's all it is) is that this is exactly what she meant. Mothers worry about every little thing concerning their children, bro; they're hardwired for that
. Ask SAR! Your mother probably doesn't see this as invading your privacy concerning sexuality, she more likely sees it as an expression of concern about who you are hanging with and how these other people might "influence" you. Will my son become a Goth? Will he "catch" HIV? etc.
So what to do? That depends on your relationship with your parents. If possible, I think the best thing to do would be to find an opportunity to discuss this with you mother. Don't raise it at a time when you are angry or she is climbing the wall over something else; find a quiet moment when you both have time and some privacy and just bring this up. Try not to make it an attack. "Mom, I know you worry about me but there are some things we need to talk about because they are really bothering me." Something like that.
What you say in your post above is more or less what she needs to hear from you. Tell her how you feel and what you need, but respect her side of the issue as well and be prepared for a frank exchange of views and concerns across the board, including the issue of sexuality, which seems to loom over the whole thing.
This is just how I see this, and I should tell you that when I was your age I talked easily and openly with my mother about many subjects. We differed on many things, but she always respected my right to my opinion and my own ideas. But even if your relationship with your mother is less open, it is probably still a good idea for you to express how you feel. You both need to find a way to make the shift in your relationship so you can communicate with each other still as mother and son, but also as adults.