I personally do not profess to claim sufficient knowledge and wisdom to dictate to others what they should or should not do behind closed doors in private.
Is it my business to do so?
Was this a private or a public setting that you witnessed this behavior? (If I may be permitted to presume you did not participate?) If you did not witness the behavior, how do you come to know of it? If it is purely hypothetical, why should it concern you (or me) (or anyone else)?
OTOH, a literalist would might point to Genesis 1:28...
It is a hypothetical question. Do you see any potential problems with everybody havinq sex with everybody else? Are there benefits to monoqamy?
Who WOULD have sufficient knowledqe and wisdom to say what is best for people to do or not do?