Homosexual superheroes are not new. The Authority is a pretty popular title, and Apollo and Midnighter are a gay couple.

The (sort of)very short answer is that in Japan there was a huge rage over Macross in the early 80's. Gundam was another huge success amongst others even a bit earlier. Due to Harmony Gold's import and reworking of Macross for American audiences, Macross made pretty big inroads into the US as wellas Robotech. Soon after the Japanese produced Transformers initially for American audiences (although they also reworked it a bit for Japanese release, sort of a reverse of the Macross/Robotech process). Transformers is much more episodic and 'robot' focused. Macross was more arc driven and character focused (ala BSG or B5 and the like).