There are many reasons for dysfunctional sex lives. The main problem is boundaries for most people, both for those who are very promiscuous and for those who are very monogamous.
You have to know what works for you, physically and emotionally.
Unfortunately, for most of us, getting to know ourselves in regard to sex is a matter of trial and error, (although some aspects of that trial and error can be profoundly satisfying).
I think there is only one truly moral issue regarding sexuality, and that is taking responsibility for all of its consequences - both the physical and the emotional - both for yourself and for your partner, and for the possibility of pregnanacy, with its limiting/limited options.
Before engaging in sex, both partners should agree on how to cope with any unplanned consequences and both should expect that the advent of pregnancy can cause people to change their minds and to have sudden and unexpected emotional (and financial} demands.
Only emotionally and financially stable, empathetic adults can really cope with all that.
The bottom line, whether you choose to have multiple partners or to be monogamous is: know yourself, be responsible, and have fun (without being overly judgmental. "Judge not, lest ye be judged").
Happy Sex to y'all!