If you look at the ancient societies of Rome and Greece, they didn't exactly live by todays western societies standard of one man/one woman relationships. Lovers of both sexes were often taken as the norm, and it was totally acceptable, and 'swinging' as we term it today was common.
I do actually remember reading something similar that also backs up what I've just said, so I'll see if I can go and find where it was and post the link!
Thanks for sharing, I found some of the other things they spoke about very interesting too
Cheers, Mrs K&T