It's kinda weird that it's something that needs to be pointed out. That's prejudice my friend. Pip's sexuality is of absolutely no importance what so ever.
What I'm saying is, that you seem to think a character's sexuality somehow defines that character enough that everyone needs to be told about, and that's just wrong. Pip doesn't try to hide her sexuality at any point, but neither does she throw it in anyone's face. What's important isn't whether or not Pip is into mares or stallions, it's how the individuals she chooses to hit on react to her advances. Make Pip a guy and the story doesn't change. Make Velvet a guy and the story doesn't change. Make Homage a guy and the story doesn't change. Make Calamity a girl and the story doesn't change. It's never been about preferences, it's about relationships. Being gay doesn't make you more or less of a person. Any special situation that you attribute to a character being gay can be achieved without them being gay.
Yes, it's sad that more mainstream stories don't feature a wider spectrum of sexuality, but if it's that big of a concern, just imagine that Hermine and Ron are both gay guys... or gay girls. It just doesn't matter.