Griffith sees sex as just a tool, so he isn't really gay or even bisexual. He loves only himself and his dream.>>10141086
Justified isn't the right word here. His actions were humanly understandable, is an odd way to put it. He succumbed to human weakness at the pinnacle of his despair. Guts did nothing to Griffith a normal friendship wouldn't do; Griffith just didn't know how to accept a friendship. He only knew his own ambition, it was never about the group, it was always about him and those he viewed as subordinates. All of Griffith's misfortune he brought upon himself.
As for Caska, she was never Griffith's girl, and I'm sure he never saw it that way.