Is the reason it has lowish reviews due to the fact that

Sorry, I just don't see it. 6.9 is a mighty high rating for a 33-year-old romantic comedy. Most people gave this 7 or higher; I gave it 9/10.

I suspect that more likely, the minority who didn't care for this didn't like the slightly "stagey" aspect of the adaptation of the play to the big screen, rather than the theme of infidelity. But I suspect most, like me, were OK with both aspects.

I think it likely has low scores because most IMDB users only rate new movies. You'll note lower scores, and lower total votes, for older movies that were made before IMDB existed.

I rated this 10/10.

Possibly. I think it's a typical 1970's film; over analyzed, over indulgent, over directed, though not overwritten.

It's a film about how two people kept a fling going on with people other than the people they were truly in love with.

The actual infidelity were the marriages they were in.

it might be, though for me it's more that i find the story too unlikely. these people are happily married and apparently quite satisfied with their lives, yet they decide to committ adultlery together once a year, for no very good reason that i can see. it just seems too improbable.

The film hasn't aged well so although it may have been to the standard of the time it was made it is rather slow and boring now. So if you were rating the film now, especially if you are younger you would rate it low.