There's never a shortage of inspiration when you find out the interesting ways of many real-life people.
The only danger is that often people will accept highly unusual things in real life but reject exactly the same thing as implausible if they encounter it in fiction.
What prompted this musing was an article I read on the website of the London Evening Standard. It includes a quote from Olivia Fane's book, "The Conversations: 66 Reasons to Stop Talking":
The rules were: when my husband was abroad for a period exceeding two weeks, he could have an affair providing (a) he had my permission, and (b) the woman didn't live in England. The quid pro quo was that I should be allowed 10 snogs a year and one two-week affair every other year, providing the location of the aforesaid affair was Stoke Newington.
I assume they lived in Stoke Newington, not that it was a particularly sexy place for affairs.
And if you're not English you may not know that "snogs" are sessions of kissing and cuddling that stop short of actual sex.
Alas, this very organized approach went wrong:
What went wrong with my first marriage was that we broke our own rules. My husband asked my permission while abroad for a short affair but it happened the woman lived in London.
It confirms that inspiration for quirky characters is as close as the nearest newspaper.
PS: I wouldn't suggest recommending this kind of arrangement to your spouse or partner unless you're pretty sure it will get a positive response.
(If you'd like advice from the best classic and modern writers on how to create great characters, get a copy of my book, "Your Creative Writing Masterclass." It's published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)