You're right that cheating is always a choice @ShagMeRiggins. I was a bit distracted there but nodding to pp's point that, for some people in abusive relationships, an affair might appear to be the best or safest way out - securing the support and protection of another adult before leaving the abusive one. Obviously not actually the best or safest way to go about it, as their judgement on the desirability of the affair partner might well be skewed by their sense of immediate need.
I do stick with my point that the capacity to cheat forms early and is part of a person's psychological make-up and moral framework. Thus the more interesting question is not 'when does someone stop being regarded as a cheat / affair partner', rather 'when did that person become a potential cheat / affair partner' and how might others spot that.
There is not only one kind of person who cheats, far from it. Some are weak and easily led, some thrill-seekers, some emotional incompetents unable to navigate the complex issues in their current relationship who believe the magical powers of a new squeeze will cure those for them, some never committed or easily bored, some resentful of their spouse and desiring to punish or undermine them, some overcome with their desire and thinking with their genitals, some likewise and prone to fantasising, some miserable but passive, allowing themselves to be 'taken away' by a white knight / princess rather than acting to end their unhappy relationship, some ruthless in pursuit of their own wants.
In every case, ending their current relationship is the right thing to do. Then looking around for someone new, or developing things further with a friend.
In every case a cheat chooses to do the wrong thing. They have a choice, they make their choice.
Portraying affairs as what 'normal people' do, as something that 'could happen to anyone' (what, passively, without warning or choice, like an asteroid landing on your head?) is what cheats and potential cheats tell themselves, to make themselves feel better.
Then they befriend the people who agree with them and form a 'we're all normal because we're the same', morally deficient social bubble, pat each other on the back, congratulate and commiserate with each other.
At the same time, there are many other social bubbles, populated by people who regard cheating as abhorrent, who would put themselves though a lot of effort and discomfort, either working to save a relationship, or going through the pain of leaving what might appear like an ok relationship, rather than cheat. Again, they befriend likeminded people and reinforce each others' social and moral norms.
That's what I've seen in the world.