I'll play devil's advocate here I guess.
There is also the simple possibility that his depth of emotion for you has changed. He may be falling deeply in love with you and, being monogamously wired, he is following his nature. It is like the transition between casual dating and committing to one person in the usual path of monogamy. For him it might simply be a natural progression based on his nature. A monogamous mindset does not always imply wanting to possess someone. It can also be about two people committing to each other in a very specific way based on mutual expectations.
The key here is seeing if there is a way to bridge the gap between his progressing expectations and yours. Society or environmental conditioning is often used as a scapegoat for people not being confident enough to follow there own path. I fully agree that the external factors of environmental conditioning play a part in many relationships but certainly not all. Don't assume he is just bending to the expected norm is all I am asking.
Hopefully you will find a way to progress in your relationship but alternately, hopefully you will be able to let go if there is no way to meet your natural needs and his.
Best wishes, take care