The same way a lot of people--male and female, straight and gay--do that despite having good husbands, boyfriends, girlfriends. Humans are tempted and depending on how much they are tempted or how down they feel or how weak they are, they give in.
I suspect this guy was never tempted ever before. He always had this lovely person he was happy with at his side and temptation was not an issue in their limited younger lives. Now, life is wider, more people, more temptations. HE's alone (he says it himself) and he feels out of place and he's been put upon by the girls and then humiliated by a dunk in the ocean. Emotionally, he is feeling out of sorts, down, a little lost. On top of that, here's this hot guy who is also gay. The temptation hit hard.
It's wrong. It's sad. His partner, who was also finding this man sexy, has been faithful.
It's clearly a bad, bad, bad, bad decision, and even he knows it. But this is the human thing: we do fucked up things, irrational things. Anyone who thinks humans are rational hasn't been paying attention to history or the news or their neighbors or their kin. We like to think we are rational, but we move on emotional more often than reason at many times of our lives. What we feel, not what we know we ought to do.
Or we'd never have religions training us and confession as an act of transformation or therapy and counseling. We fuck up.
Natsuki fucked up.
We often act on emotion--whether positive or negative. And though I am so angry with Natsuki, because he has a lovely relationship he's about to majorly screw up and a partner he's going to hurt horribly due to this, I get how the author is setting up his temptation and weakness and emotional immaturity. This guy has been with one partner since he was a young teen. This is the wider world. He's got temptations like never before. And he feels alone (he's probably been stuck to Arata's side for ages and never had to be out in "the woods" literally or figuratively with a hottie while feeling like an outcast dweeb.). Sexual desire for a young man is a killer thing. Hormones, youth, heat--it's hard to fight that and a lot of folks do not, they fail.
The statistics on the infidelity of heteros is bad enough, but when you see the stats on the promiscuity/infidelity among gay couples, it shows you that temptation and giving in is not abnormal, but the norm. Natsuki is not an outlier. He's pretty much being the norm. He's experimenting with something new and irresistible.
I feel awful, because when people are in love and together for years, you wish them well and hope they keep being happy and in love. But most relationships hit hard spots in the road and you either give up or fight to get past it. I'm big on fighting to get past it if it's WORTH it, and only the people involved can decide if it' worth it.
I suppose these kids will decide it is. But it's gonna be tears and agony first...
And I think you missed my point about religious training in confession and reconciliation. Most religions I know have a path for people to confess their wrongdoing and fix things (atone, reconcile, make peace). If Natsuki and Arata go through that process, they can be fine. Natsuki's cheating is a wrong. It's a hurtful thing. It's a sin, in many people's eyes (ie, cheating on a partner). And yes, there are many people who see homosexual relationships as sinful, but that's not the point *I* was making.
I was making the point that BECAUSE human beings routinely do shitty things--lie, cheat, steal, beat up, abandon, etc--that societies and religions in them have ways to deal with that. In, say, Catholicism, you go to confession and admit you did something wrong to get forgiveness from God and then you are responsible for rebuilding the relationship that was harmed or returning the thing stolen or whatever else is the reconciling thing. The harmed person is then under a duty to forgive and participate in reconciliation (in the Christian religion), in order to return harmony to the community.
If we do not have ways to do this--confess, atone, repair, return harmony---families and societies cannot function.
That's why, knowing humans fuck up with regularity, we have to have compassion, mercy, forgiveness, and love. Otherwise, guess what, hope is lost.
For people who say that Arata should dump the hell out of Natsuki-that is one option. But is it the option that would make them truly happy? I mean, what if Arata regrets it a year or more later. Regrets he didn't forgive? What if Natsuki regrets it if he doesn't confess and try to atone?
Now, if both do wrong things, then both have to apologize and work through it--whether they go separate ways or stay together. But humans will never, ever be perfect. Never. You will hurt people. I will. Willingly or accidentally or repeatedly. We will. So, we had better learn to understand that Natsuki being shitty makes us furious, but it also is unrealistic to think WE would NEVER do such a thing. Clearly, many, many people have cheated on partners. MANY. Forever. Natsuki is the face of many of us.
This is why I can't hate (nor am I condoning cheating) because I understand the why's. It's just inevitable. Growing up in an Asian country and imagining that it's probably not that different in Japan, most high school graduates aren't entirely mature, well probably in every country, BUT I don't know, it's just different in western countries.
I knew exactly what, why and how Natsuki felt and fell into cheating because I immediately imagined myself to have thought the same thing if I was in the exact same position as he did. Same situation, same age etc. Well, my moral compass will win on stopping me from acting on it but I would be definitely think it.
Secondary education in Asia is just so....strict? Can't find the right word but you don't get to be as wild and adventurous to experience various emotions to equip you about the real world. In my country, HS culture has changed over the years, it has become westernized, I'd say, but back in the day, we were sheltered (I was in a private school so maybe that's why). So I imagined right away what it would feel like to have your first love since middle school up till college and never really breaking up and never really experiencing sexually interacting with another person. The temptation will definitely be strong.
There's also many facets to cheating that I personally have come to terms with, but that's another topic.
I'm just so impressed how Sensei wrote this story. It's complete and logical yet just in one volume.
how he could do that, while he has a good boyfriend