He was 49 and she was 23? Gross!

She was older in the movie. They're actors portraying roles. Don't get hung up on their real life chronological age, it's not germane to the story.

Additionally, when Wendice is talking to Swann, and describing his failed marriage and the discovery that she'd been seeing Halliday, Tony says they were school friends -- so they're all supposed to be chronological contemporaries.

Are you 12?

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They were having sex in real life. She didn't mind men to be a lot older then her in real life. There have been older women I've thought are very attractive in real life and actresses.

It matters how? 23 is old enough to make consensual decisions.

There's nothing "nasty" about it. This age gap has always happened in Hollywood and always will.

However, if you look at Grace Kelly's leading men in the 11 films she made, almost all were old enough to be her father. Grace was born in 1929 (in November; she was 24 when actually filming Dial M). Among her leading men, their birth years:

Gary Cooper, born 1901.
Clark Gable, born 1901.
Bing Crosby, born 1903.
Ray Milland, born 1905.
Cary Grant, born 1904.

Even Stewart Granger (born 1913) and Alec Guinness (born 1914) were at the edge of what you'd normally find as an age difference of a couple.

The leading men closest to her age were William Holden, born in 1918, and Louis Jourdan, born 1920.

Grace had affairs with almost all these men, and it's been suggested she was looking for a father substitute. In fact the one she came closest to marrying was Ray Milland. But Milland's wife was very popular in the Hollywood community and a divorce would have hurt his career, and Grace's father opposed the union, so both backed off.

It's not gross.

Yet the OP has "romance" in their user name. Rather ironic.

Milland was born in 1907 (not 1905 as his IMDB page incorrectly states), so he was around 46 when making this film. And I don't think anyone here has complained about the absurdity of a middle-aged actor playing a character who only gave up professional tennis a couple of years ago.