To those who subscribe to and live by conventional norms of man and wife, yes this is strange, perverted, and usually unacceptable behavior. For those married to each other who define their own rules and acceptable behavior within their marriage, it’s no one else’s business.
My wife and I have been there, for the experience. Mutual agreement. She went out and met another man and then came home to me. And vice versa. We haven’t done it often, and we didn’t/don’t do it as a consequence of any open relationship. It is as permissive situational circumstances.
We are and remain happily married for years, many years, now. Our marital and emotional bond to each other could not be stronger and more sexually understanding.