This is a significant development for Doctor Who as this marks the first time an ongoing companion is openly gay. Yes, you might argue that something was going on with Nyssa and Tegan back during the time of the 5th Doctor but that only lives in your fan fictions. Yes, Clara Oswald may have made out with Jane Austen and Amy Pond literally flirted with herself but no, this is different. This is not an off the cuff joke but a serious thing.
Here's what Pearl Mackie had to say on the subject:
“It shouldn’t be a big deal in the 21st century. It’s about time, isn’t it? That representation is important, especially on a mainstream show. It’s important to say people are gay, people are black … being gay is not the main thing that defines her character — it’s something that’s part of her and something that she’s very happy and very comfortable with.”
So good on Doctor Who for making history. I'm looking forward to seeing what Bill brings to the TARDIS when she joins the Doctor and Nardole in 2 weeks.