Can you help me understand this situation in the series "Frankie and Grace"?

It’s a wonderful series with Jane Fonda and Lily Tomlin. Franky’s husband is played by Sam Waterston, and Fonda’s by Martin Sheen.

For those who’ve never seen it, it’s about those 2 couples, both in their 70s. They’ve been married for 40 years. The husbands have worked together for many, many years. The wives are wary friends.
One day the husbands announced they’re leaving their wives to be together because they’re gay, and they really love each other. They’ve been trysting for 20 years.

There is just one thing I have a problem with. It really bothers me a lot, and I’m hoping that talking it out will help.

Franky and her husband obviously share a deep and abiding affection for each other. It looks like love to me. The kind of love everyone wishes they could have had for their whole lives. They are “soul mates.”
I just don’t see that same spark between him and his lover.

Why is he leaving her again? It can’t just be for sex, can it? I’m almost 60 and I would certainly value the depth and quality of my relationship with my husband, especially if I’d shared my whole life with him, far more than the sex. How can he break her heart like that?

And yes, I know it’s just a TV show. But I’m curious as to why they wrote it that way. It’s breaking my heart.

And Fonda looks more beautiful than she did when she was 20! Not fair! Lilly doesn’t look much different either. >_<