"but the writers suddenly turned her gay"

Still missing the point. She wasn't "turned" "gay". She met someone she was attracted to and they had a relationship. Pretty much what she'd been doing throughout the entire series.

And if you'll recall, she slept with Roy again after that. Was she "turned" again? Or does she simply follow her heart?

Keeley's arc in season 3 was basically that she slept with the boss. Which became a problem when the relationship ended and she effectively got fired. But that, in turn led her down the road towards standing on her two feet.

If Keeley's backer had been a man and they'd done the "traditional" slept with the boss thing I don't think anyone would have complained that the storyline didn't make narrative sense.

The underlying theme of the entire series is pretty much about seeing people as people. Race, gender, orientation, whatever, it doesn't matter. Why is Keeley's relationship with Jack any more surprising than, say, Rebecca's relationship with Sam?