They didn't make Varus gay. There's nothing in his new lore that supports that statement. They changed him from a man who allowed himself to be possessed by the Darkin to a Darkin who possessed the bodies of a gay couple. No mention of his sexuality or any part of his character other than him being beautiful, sadistic and out for vengeance.
Rather than having a PC agenda, it seems to me more like they were already planning to rework his character to give all the Darkin a concrete, running theme (I.e. they all reside in weapons and possess humans). The gay couple just seems like a convenient opportunity to appease the people who have been begging for LGBT representation to me.