I think using the term "lifestyle" or "culture" is fine, at least for me.
I mean, why not? We use those words to describe everything else in society. I have friends that talk about the "Bohemian lifestyle" or the "vegetarian lifestyle" or the "gay culture" or "Black culture" or "liberal culture" or "geek culture" or how being a professional athlete creates a whole different lifestyle than, say, being a computer programmer.
For myself, I disagree that the only thing that makes poly people different than mono people is that poly people allow for more loving relationships. For me, just the fact of allowing for more loving relationships and partners also denotes a radically different way of viewing the world and living in society than what I have always been used to; on a real practical level, I don't understand how me and PLove welcoming new people and their energy into our lives would NOT change our lifestyle in some pretty major ways. To me, its like saying that the only difference between a social conservative and a liberal is how they vote.
Who I love, how I love, what I love, and how I form partnerships all speak volumes about how I live in the world. Again, this is my take on the words for myself and how I understand them. Not saying that lifestyles and cultures don't overlap or blend but there's a reason why we have the words "culture shock", "culture war" and "lifestyle changes" in common parlance.