I'm a female software engineer at a medium sized startup in the USA and I've always been a bit masculine. It's never really bothered me.
I recently started working on a new project with another team in the company. I got to know one of the colleagues that I've been tasked to work with pretty well. We work a lot together and she was initially, for the most part, nice to everyone. She's more junior than me in the company (and significantly younger, I believe).
One day we went out to lunch together and she confided in me that she is a transgender woman, thinks I am a transgender man who just doesn't know it yet, and she's in love with me (at least, me as a man, as far as I understand). I was, understandably, flabbergasted. I tried my best explain nicely that I am not trans, I am a bit of a tomboy but am perfectly happy with myself as I am. She was not convinced, and is determined to prove that I'm a man.
Needless to say, things have been awkward since. I brushed it off and hoped she'd never bring it up again and we'd get over this little incident and move on with our lives.
Unfortunately she's bought it up every single time we're vaguely alone in conversation (which is quite a lot because we're working together on the same project). I've resorted to avoiding her, or making sure someone else is around.
I was happy to just ignore her and move on with my life (I'm married, and she knows that!), and I really just want to do my work. But now she's resorting to making comments to my colleagues, (like "don't you think Stacey acts a lot like a guy?" and "Maybe we should call Stacey 'He' or 'They'") and people are starting to notice.
I asked her politely to please stop this, and she got really defensive and said that she was just trying to help me, and I'm "brainwashed by the patriarchy". I got more stern and said my gender is none of her business, and she called me a bigot and anti-trans and "if I was for trans people I would be proud to come out and say I was one".
I'm really stuck for what to do now.
- I really really don't want rumours going around that I'm a guy. Because I'm not. And some people have already started to ask me about this.
- I don't want to come across as some bigot and make a big issue out of this. She obviously has some issues and I really just don't want to be part of them.
- She's making things more and more uncomfortable and I'm feeling harassed by her and I don't know what to do about it. She's threatening to tell people about my "dirty little secret".
- I'm really hesitant to go to HR (we have one) because I know that the world is really pro-trans these days (which is fine) and I don't know how the company will handle it ("HR is not your friend" and all).
How can I make her stop?
 I was a tomboy growing up and it's just how I am and I've come to terms with it. I don't wear makeup and usually just simple clothes but it's very obvious I'm female (I have long hair and obviously feminine characteristics).
 I have nothing against the trans community, I get that people are trans and I respect their pronouns and treat them as I do anyone else, but I'm not trans myself.