@Asuitcase
It's perfectly possible to "truly love" someone and for that true love not to last a lifetime...
Then it's not true love, true love has to be protected and respected constantly, it's a work in progress, it not there one day and gone the next, if something is special then you are more careful of breaking it, true love has no time scale, it means eternity.
I think you're confusing two things here and your outlook is very very utopian. There's no such thing as an objectively measurable "true love", it's not a chemical element which can be measured via a PH test. By definition, love is a subjective and mutable condition.
There are lots of different kinds of romantic love. It is possible to feel total genuine, selfless, all encompassing love for another person at a period in time but for that love to eventually fade away or to get to a point where it's no longer healthy or constructive. That doesn't invalidate the fact that there was love there. I have truly, deeply loved people in the past who I rarely think about now. The fact that they are no longer the dominating force in my life does not mean I can't claim it was love. Love is more than just an endurance test.
It is also, as a separate point, possible to be sufficiently committed to your marriage that you prioritise it over all other else in your life and will stick to it through thick and thin and in the face of temptation. And many people do this. But that isn't necessarily just about love. It could be love or it could be your personality, or your religion or just the fact that you have too much invested in your marriage (financially and/or emotionally) to walk away from it.
That's not necessarily for everyone though. Marriage and LTRs can be life affirming but they are also a big burden on many people and bring a lot of baggage and commitment which a lot of people aren't really designed for.
Cheating is obviously never the best way out of this. But I can't help feeling that part of the reason people cheat is our over-investment in the notion that "love" underpins the whole structure of a relationship. Relationships should have love at their core but there's a ton of other parameters involved too: money, children, logistics, friends etc. Most successful long-term marriages take on a life of their own after a while and at a certain point they become about more than just "love". And of course historically most marriages weren't based on love at all, they were based on financial arrangements made to benefit the broader family. Love was an afterthought and a bonus. This isn't wholly disastrous, and it does keep people together. But the notion of a marriage is about a lot more than just love and it's very difficult to dismantle.
I think if we were a bit more honest about the role that "love" plays in the edifice of marriage we might be able to be a bit more pragmatic about things when marriages stop working for us, lose some of the hysteria and recrimination. The whole notion of "cheating" could become easier to deal with if we accepted that not everyone is cut out for a lifelong endurance test of this kind and we would be much more pragmatic about changing the arrangements to fit our needs.