My case is already universalized. You are the one coming up against the status quo. Secondly, I'm sure you know of anecdotal cases where people are happy sleeping around, but it defies the sense of community people who think like me like to create and maintain. Sluts and Manwhores get shunned. Lastly the family overides a person's individual happiness, and part a what religion is and does is to help ease the pain of losing the right to run around screwing everyone in sight, in favor of playing a the role of Mother, Father, husband and wife. The very word husband means banded to the hussy. Marriage is an agreement to value your children over yourself.Again, you conflate promiscuity with cheating. Hint: not everyone dates the people they have sex with. Not all promiscuous people date or have sex with multiple people at the same time. Another Hint: there exist a certain number of couples with open marriages, at least one of which that I know of that has children. They do more or less the same as other couples. Not everyone conceives of sex and family in the narrow way you do, and thus your views cannot be universalized.
You have put no arguments forward as to how it is a character flaw, so no it isn't obvious to me. I know of promiscuous people who have families (polyamory and the like) that have loving family lives, and monogamous families which are miserable. You have put nothing forward to make promiscuity a character flaw in respect to the family or anything else.
Sean