Earliest game to feature globohomo that I could find was probably "Caper in the Costro" (with little documentation, but was supposedly distributed via gay bulletin boards) or "Circuit's Edge" (1989) which featured a veritable slew of degenerates. Both sourced from Wikipedia, of course.
Obviously, it didn't become more common until the early new 10's, with it only appearing sparingly in the naughts with stuff like the Ballad of Gay Tony (2009) leading into the wave of homosexuality in the 2010's.
I imagine the best time to pin down as when it actually swapped over to the mainstream was when Naughty Dog retconned Ellie into being a lesbian in TLOU: Left Behind (2014), although it was obviously building up prior to that (although not as noticeable, perhaps?)
So, basically within the last five years, in other words.
For whatever this excessively autistic analysis is worth.