I had the same problem, but with economy class. It seems that Etihad blocks all window seats when you check the online seat map at time of booking. What worked for me is joining etihad guest and selecting window seats as preference. As soon as I booked the flights I was automatically assigned window seats.
Second option is to call them and request the desired seat, i've heard their staff has access to the full seat map.
Third option is to login as soon as online check-in is available, then you are able to pick from all available seats including the ones that appeared blocked before that.