People are too stuffy and possessive. Maybe some people can just sleep with one person for all of their life, but not all. We should stop the hypocrisy, especially if you are from the school of thought that it is about controlling women's uterus to make sure men aren't spending resources on another man's genes.
That's what divorce or breaking up with somebody is for.
Otherwise it's lying and being dishonest to somebody who you profess to love.
That's not stuffy sexist thinking. It's being nice to other people.