My boyfriend thinks my other relationships are awesome. His wife thinks his relationship with me is awesome. And so on.
Which is an issue independent of open relationships. There's not a lot of research on open relationships out there, but there's none at the moment that indicate any harm to children exposed to them.
Also it must be pointed out that the majority of those girls you know of probably have monogamous parents, in name at least. Serial monogamy and cheating are both things that make monogamous a bit more monogamish.
What do you mean by the bolded, sex in front of kids? Obviously not ok. A random string of people in and out, also not ok, the same as it wouldn't be ok with a single parent dating one person at time to constantly bring that kind of unrest into children's lives.
But as pointed out, that happens in monogamous relationships, not just polyamorous or open ones. You're assuming that an open relationship = open casual sex where in reality there are a wide variety of relationship styles.
I think open relationships are as a whole, morally neutral. I feel the same about monogamous relationships.
For someone who is unsatisfied with one partner, and open relationship can be beneficial because they will be able to have intimate relationships, but not harm others by cheating or lying about his or her ability to be monogamous. For people who are completely or primarily satisfied by one partner, then monogamy or monogam-ish relationships may work best for them. They wouldn't necessarily feel happy involved in an open relationship.
So I suppose I could argue a moral benefit, but I don't actually think it's necessary to have one because relationships only have moral value to me in their ethical and honest nature, not the shape of the relationship itself.