Acceptable, and I have yet to see a good argument against it.
Polyamory, if you scale it up, will trend heavily toward polygyny. This is an immutable fact of how humans are wired.
The inevitable result is a society in which there are few available women and lots of unlucky single men competing over those women. This situation is going to be much more dysfunctional, for several reasons:
1. Men often compete for women through things like violence and ostentatious displays (rather than by, say, seeing who can cure the most diseases or something)
2. The energy and resources used to compete for women could have been better spent doing something productive
3. This extra competition lowers trust
Additionally, the reduced attention children will get from their fathers will take a toll on long-term development. Plus, the size and quality of the gene pool will be reduced.