An interesting exploration of infidelity and the forgiveness of it: on both a personal level as well as how the community around you forgives it, I think. The fact that the protagonists are gay is somewhat incidental if you think about it as a broader analogy for anyone bound into a partnership that rubs against the grain of who they are; the root causes of unhappiness in relationships are diverse, but their emotional resonance is more or less the same. We either act or we don't. We do questionable things to block the pain, we ignore it and go numb, we do, we don't.
I don't mean, of course, that the author doesn't personalise the story to fit the characters -- it's very much about both Andrew and Nathan and how their sexualities identify them in different ways over the course of their lives. I liked very much much that the author put them in different places in their respective journeys but that both were still struggling with aspects of their sexual identities. GFY is only really believable for me if everyone's a bit confused for a while.
The writing is fairly straightforward, which mostly works. There was enough drama in the plot without having to enhance it with overblown imagery, though the author writes the sex scenes with loads of emotional markers so it's not like she couldn't have gone full metal adjective. I respected that restraint; angst usually equals hyperbole in the romance world. I'll grant you that in parts it gets a bit wooden for lack of a little floridness, but I could live with it.
There were some significant issues with pacing, which made things less satisfying for me than they could have been. Things moved way too fast: Nathan admits -- and acts on -- his attraction to Andrew so quickly that it entirely changed the rest of his story for me. They fall in love over the course of a few paragraphs. Nothing is as fleshed out as the situation seems to demand: a secondary love interest develops almost spontaneously as Andrew and Nathan break up for the penultimate time. The author hurries us through details that, for me, would have been the meat and bone of the whole thing -- and then expects the reader to be invested in the emotional processes of characters who are presented to us as snapshots in time.
There were also some very odd choices made in terms of the communal approach to the characters' extremely complicated and fractured relationship: the people around them were almost completely accepting and tolerant of their approach to those complications (in almost every instance, infidelity). It's a tricky one, given the need to reflect some sort of sympathy for the protagonists and the pain they're experiencing, but on the whole I think making everyone supportive -- either directly or obliquely -- was the wrong choice. The background story (Nathan having to deal with the responsibility of a wife and two small children/Andrew being asked for a divorce and the subsequent fallout) lost a lot of depth for that very real element of social reflection being eliminated. Everyone who was intolerant of the behaviour was either definitively bad (Andrew's extremely religious mother), or a threat (Nathan's employer).
I've never thought of infidelity as something you can approach didactically, personally. There might be reasons for it and reasons you might sympathise, but as an action it has consequences and those consequences almost always affect the people around you. Nulling those consequences with universal acceptance sort of defeats the purpose of bringing it up in the first place, to my mind.
I enjoyed this on a certain level. Technically I think it had a number of flaws, but as a simple story and an exploration of a premise I think it did what it wanted to, and did it well.