I'm less surprised by Adrian's attitude than many here, it seems. Remember, his parents are poly. He grew up in an environment where love did not equal sexual exclusivity. Why are we surprised that he accepts that as normal?
Mostly because in the beginning of the comic Erin was getting mad that Adrian was getting it on with Dani, before the Night of the Watch. It wasn't exactly established as a norm of the setting prior to things taking a turn for the poly.
Plus, he's been in a poly relationship for quite some time now. Apart from - and as well as - Erin, I believe Adrian has had sex with:
*snip*
Yes, there are all sorts of other factors at work here, but he'd have to be a total arse to demand that any of these women remain "faithful" to him. And the whole point of Adrian is surely that he isn't a total arse.
I think there's a difference between what is basically a fantastical mind-controlled harem sort of relationship and a "realistic" poly/open one. I was expecting the former, unfortunately.