Just because your monogamous friends are happy and fulfilled doesn't mean your slutty friends would be happy if they lived the same way.
Humans are varied.
Talking to my Tindersex-addicted friend about his behavior - which has been very destructive to his own life - he believes that it comes from a place of trauma as a coping mechanism, not simply a lifestyle preference. In a similar vein, many women I know who only had uncommitted relationships in their twenties are now in their mid 30s and interested in settling down but are having extreme difficulties in finding a partner.
Note that these same friends also have stable, fulfilling (non-monongamous) relationships in their lives.
I think that's why I bristle when I see people equating slutty behaviour with lack of fulfillment or trauma. I think that sex absolutely can be an unhealthy coping mechanism; but so can alcohol, yet many people manage to use alcohol in a fun and safe way.
I think that presenting commitment and sluttiness as the only two options is harmful, and it keeps people in destructive sexual patterns because they don't think they can have broad sexual exploration at the same time as serious relationships, so they avoid relationships altogether. It takes a lot of work, but you can have both! And I think that the more we become comfortable with that as a society, the healthier we will be.
This isn't backed up by research which seems to suggest that the more sexual partners you have, the worse your relationships are in practically every dimension.
This is an area where it's very difficult to show cause and effect (since you can't exactly do a randomized controlled trial) so I would be skeptical about drawing conclusions that lowered relationship quality is caused by having more partners.
Non-causal and uses marital happiness as a proxy for actual happiness though.