If the story is as presented, that's a shame and I'm sorry that happened to you.

That said, it's really a story of doing the wrong thing at nearly every turn. Why date someone on your team, you admit you knew better.

HR is not your friend or there to help you, they are there to protect the company from litigation. A coworker being short is not going to raise any flags. Her claims of harassment, in a company known for it, absolutely is - regardless of whether its actually true or not.