*But if he's with a 50-year-old he is demonstrating that he finds 50-year-olds attractive? So why is there no speculation about what thoughts he has when he sees other women of a similar age?
Why is there only this problem when his partner is 20ish?*
Because if he found contemporaries attractive, that would not be unusual. Why would it matter? We've all looked in the shop window - there's a difference between looking at the new shinies and taking them for a test drive.
But damned right I'd have that thought, Hang on...if he fancies someone a generation younger, then what was he thinking when teenage daughter had her mates round... Or he was chatting to that Newly Qualified Teacher straight out of college at Parents' Evening... Or that young woman who's new at his work...
That is a perfectly reasonable thing to 'speculate' where he'd actually draw the line. My husband's in his late 50s. If he fancied a woman in her 40s I wouldn't find that weird. If he fancied a 28 year old - as I have a 26 year old son - I'd find that incredibly gross.
If I was the Mrs of Mr Wonderful, or come to that, one of his friends,or the woman down the pub, or his kids' teacher or their new neighbours when they move into their bijou divorced man on midlife crisis flat... everyone who will know about this when the shit hits the fan, basically... I would 'speculate' or rather, as that's a loaded word, 'wonder' what else would go through his mind, when around young women - not even attractive young women, possibly if the 'young' is the pull.
OP, if you're still reading this (you are, right?) find out what form he has. You won't be the first.