It's fair to say that there's a lot of people who simply don't understand the facts when it comes to queer identities, including homosexuality, bisexuality, and so on. Yes, it's natural. No, it's not a choice. And we're not all going to burn in hell now that same-sex marriage is legal.
One thing a lot of people don't understand is that, even if they think heterosexuality is the norm, it lies on a spectrum, and very rarely is is a conscious choice. While it isn't quite the same as a physical trait like eye color, it is the sort of trait one develops unconsciously. We can choose who we love about as much as we can choose what foods we like and don't like, what smells appeal to us, or any number of traits that make up who we are. However, strangely, some people seem to think a lot more choice and thought goes into being queer, yet never stop to think about all the thought that went into their "decision" to be straight.
One professor is pointing these things out to students with a very pointed quiz, turning common questions people ask gay people on her straight students who are probably not used to having their sexuality called into question or being shamed for displaying affection toward their romantic partners like holding hands or kissing, a very common concern same-sex couples face.