@rosestar I've been married twice, and I've lived with several women before that. I prefer to live alone, though I don't like being alone. This is a psychological thing, and I know the reasons for it.
"Love" starts with lust. After the lust is gone, many married people want to be free.
If you can handle it, and if it is agreed on by both or more partners, whether married or living together, you can try an open marriage. It's difficult for most.
I think "love" is some kind of misnomer, but I've met one couple in my life who had an open marriage, and both of them were quite content with each other. The only "rule" I would insist on, in a case like that, is that both or more of the parties involved agree to the living situation. You could have two men and one woman, vice versa, or two or three couples living with each other. They might all have kids, but those kids could get different opinions from several adults.
There are any number of agreements or living arrangements people could consider. Much of what people think about a monogamous life is a societal expectation.
The initial problem can start with a simple sexual encounter.