Having spent 25 years working in the city and with big4/magic circle partners & teams, I've seen this scenario hundreds of times. Professional bloke marries first in 20's and has family. Then wife stops working, turns all mumsy and stops being the person she was before. Bloke then meets junior colleague he clicks with (usually, not always, before leaving miserable wife). She is suddenly all the things his wife used to be. And the new adoration will continue until of course she has kids (and starts changing too). cycle repeats ad infinitum. It is now so common that most of the big partnership model firms have a very big bottleneck at the top - whereas partners rolled off into retirement in their mid50s before, now they are carrying on because they still have young second families (I've seen instances of third families too) to support. This is why equity partner entry salaries are bring pushed down and why now so difficult to break barrier compared to 20 years ago. take a look at the 60+ partners in your own firm, OP, you'll see that many of them have second families and large age gaps with their wives. They are still there in their 60s because they need the money. You will also see quite a few who are divorced from second wives but still have young families. Ask any senior partner you trust about all of this - it's rife in every partnership group.
You see him as an intellectual equal. He will only treat you as the same whilst you stay the same free, young person that you are. That's not the equal partnership you think it is. You asked how people feel about these relationships? Well, given the exact situation you describe, and the 25 years I've spent (laterally at the top level) in my career, I'd say that usually I really struggle to see who is the bigger fool.
By the way, in general I don't have a problem with age gaps. It's just these 'professional' serial guys that get on my goat. And its nothing to do with age, but everything to do with their attitudevtoward women. They are in charge of large firms and responsible for the career development of many of our daughters, yet they think it's fine to live these selfish lives where their wives are pretty appendages to be discarded when the relationships get tough, or when they 'no longer feel appreciated' (diddums) or when 'their wives lose interest in them as individuals (wonder why). It just doesn't bode well with the kind of respect that I want for my own children - if women are seen as equal only when their lives are uncluttered (ie when they can in effect act like men), there is little hope of normal women(with kids and responsibilities) making it to the top in these orgs. Which I am sure lies totally at odds to how you think he views women.
I have seen a couple of personal friends (both magic circle lawyers) waste years and years on men like this before being discarded for a younger model, which does make me bitter and twisted, I know (and I have also seen guys like this mercilessly exploited occasionally by pretty young things). What upset my friends most was that they never had kids themselves (because their partners didn't want all the nappies again), but they still lost out to younger models in the end and were left, in their 40's, up shit creek, with little other than their careers, and no chance of kids. Hopefully your relationship will be the exception. if I were you I'd be checking out the 20 year old's reaction to you very carefully - they will have sussed out their dad's form by now, they'd be very useful to talk to.
One thing I can guarantee though, OP, is that he'll still be skiing 4 times a year in his 60's ( just probably not with you).