Door into Fire by Diane Duane was written in 1979. I think I read it for the first time in 1992. The two heroes are unapologetically gay and young, there is no stigma about this relationship. In one chapter, the parents (mother?) of one of the protagonists calls upstairs and asks "What are you boys doing?" The protagonist calls back, "We're fucking."

Again, this was written in 1979.