A great test for real love is to ask yourself if you love that person enough to let them be them self.
Do you love that person enough to love them when they don't love you?
Do you love that person enough truly want them to be happy above all else?
Those are all good.
I'll agree with those.
Also, by those definitions I have been in love.
However, the people I've been in love with weren't people that I would want to have an intimate romantic relationship with.
I mean, just because you love someone doesn't mean you want to get physically intimate with them.
Loving your parents or children is a good example of this.
Loving friends could be a good example too. Although in the case of friends you might be interesting in getting intimate with them too.

I might also say that I have indeed fallen in love with many men's wives.

Hey, there's nothing wrong with that!
I didn't become physically intimate with them. But it is true that if they hadn't been married I would have been interested in marrying them myself.
So it's not that I never fell in love.
I just never fell in love with a person who was available and willing to move in with me.

But, I seriously believe that if I had it would have lasted forever, saver for major personality changes, or other factors.
I personally haven't changed in personality in any major way since childhood. I was highly moral and monogamous-minded then, and I've remained that way throughout my entire life (at least up to today). I see no reason why I should suddenly change save for a medical problem with the brain.
If I haven't changed, and I had married a partner who also didn't change (in major personality ways), then I see no reason why would would have fallen out of love.
The other thing too, most of the people I've seen get married around me did not love each other in the way you described above. That was obvisous from the get go to me.
They were trying to CHANGE each other even before they got married.
At least that's true of the marriages that didn't work out.
I've also seen couples that were truly happy with each other for just who they were. And they are still married today.
My cousin who lives next door to me got married quite young (in her 20's). She's my age now (60). They've been married 40 years and they appear just as happy with each other today as they were the day they got married.
I have never seen them have a major falling apart. I've seen them argue, not seriously mean. But clearly pissed at each other. But it never appeared to amount to anything serious.
If they ever had serious problems I'm totally unware of it. And I've lived next door to them for the whole 40 years. I think I'd know if they had a major falling apart.