Speaking of trying to fight heterosexist language, I’m not a native english speaker and I need a substitute for “opposite gender”. I hate that term.
There isn't a great single term to replace it, but there are strategies.
To begin with, I just try to avoid it when possible and be specific about the genders I am talking about. For example, I don't use the phrase "opposite gender relationship" and instead use "m/w relationship" (or man/woman relationship).
When it comes to defining sexuality, I think like/unlike and similar/different are good terms to use. If I was to define straight, I could define it as "attraction to genders different than one's own" or "attraction to genders unlike one's own". If I was to define gay, I could define it as "attraction to genders like one's own" or "attraction to genders similar to one's own." This not only rejects the gender binary framework, but rejects the idea that people need a specific sexuality to be attracted to nonbinary people.
If I am really struggling with a way to say what I want without using "opposite gender", I will use "other gender" instead. It's not a great alternative, but it is concise and gets the point across.
