This is only IMO:
The breakup in the relationship (marital, in this case), was inevitable.
As the years went on, Ashton knew either consciously or subconsciously, there was an imperative to have children of his own.
Demi had already passed through this phase (3 children?) and could no longer have a baby? I'm wondering if she tried through invitro. Because that would make sense. She is wealthy. However, maybe she said she didn't want to go through that again. Possible.
I think he felt the only way to end the relationship was to act out in a big way--philandering. Perhaps he tried to end it sooner, and this proved to be impossible for him.
I do not doubt that the two of them were madly in love with each other for many years. In fact, I won't be surprised to learn that Ashton passionately professed in truth to Demi that the age difference didn't matter, and it would never matter, at that time.
This is not to say this difficulty happens in reverse. Many younger women marry older men. Then, after awhile the "biological" imperative begins to push them toward having children. Men can naturally conceive children well into their senior years. An example of this, would be Picasso having children into his eighties.
However, even though some men can, some men choose not to because they've already been through that, and don't want to be saddled with the responsibility of raising children again. Or, they don't have the money to be able to hire nannies, etc.
In conclusion, I think this is the reason Ashton is exiting, even if he isn't conscious of it yet.
I think Demi should be very grateful for what she has. She is living a very rich and full life.
