"Dynamic Duets" had Blaine and Sebastian, both gay, and Brittany, who has said she was bi-curious while presumably meaning bisexual. That was enough to make it not heteronormative. I don't know when the show became one where you couldn't assume characters were straight, although it was arguably the first episode. It's gotten so that characters say that they're straight not out of homophobia, but by way of clarification. By the thirteenth episode, "Sectionals," Brittany said that she and Santana had sex to several members of New Directions. In "New York," Quinn said, "I'm not that into that." She was specifying without hostility.
Characters questioned whether Kurt and Rachel were cis. With Kurt, it was about him being gay, things he said, the way he'd rather be with the girls than the boys, how he sometimes mixed a piece of women's clothing into his layers of clothes, and liking activities and having interests considered in the fictional Lima to be feminine. In "Audition," Kurt said to Mercedes, "Fashion has no gender." Some episodes earlier (in "Home"?) he'd said to Burt, "I'm a guy." But to many observers, Kurt often enough managed to queer gender.
Unique introduced herself to Kurt and Mercedes as Wade. In the same episode, though, she said that Unique was a woman. She said that as a child she'd played a game where she was Unique. In the fourth season, it's become increasingly clear that it's no game. So Kurt and Mercedes learned not to assume someone was cis, and then the show choir world learned that.
When I was in college and just coming out myself, Ellen coming out was the big thing on TV to our group, and then there was Will and Grace. I never imagined there would be a show like Glee, with so much visibility of young GLBT characters. New characters know that within the world of the show, they can't be assumed to be straight. Being gay or bi has become normalized. It's the beginning of the process with Unique. That's going to be a journey.