I think Christmas is a bit like monogamy - it feels non-optional, fixed and traditional, but it's ubiquitousness is really just a result of social conventions that can be taken to pieces and reconstructed any way you like. For some, the day has religious significance, and for some it's more about raucous drinking and indulgent eating. And for some it is both, of course. You don't have to set a Christmas pudding on fire, spend hundreds of pounds on presents or festoon your house in tinsel any more than you have to stick to one partner at a time. These things are all negotiable, even though society does its best to convince us that they are not.
But good God does society do its best to make you feel that you are missing out when you do. I've been told that I can't really love my husband if I want to see my boyfriend as well. I've been told that my daughter will grow up resentful and confused by the lack of a monogamous bond between her parents. People who don't have big, happy families are made to feel lonely because they are constantly being told how much they are missing out on. And people I know who've decided to break with their families' traditions and do something different over the holiday period have been made to feel that they've let people down. One of my siblings suggested off-handedly to our mum that her partner and children might like to go abroad for Christmas, but the resulting wave of guilt soon persuaded her to stick to the status quo. (And to be honest, I was glad, because I didn't want them to go either.) And, of course, polyamory creates yet further complications that can make doing what you want even harder. The more entangled we are in other people's lives and other people's wishes, the more complicated and fraught it can be to do our own thing.
If you've run into, or are likely to run into any of this, I obviously can't tell you how to fix it. The only general advice I have is to stay flexible. Remember that the whole holiday season is arbitrary, and so find places where you can bend to make things easier. Perhaps getting everyone you want together for December 25th is impossible, but you can treat it as a movable feast, and organise it for another day. Perhaps you could go for something close to our solution, which is to use Christmas Day as a biological family get together, and New Year's Eve for the other important people in our lives.
Things can be complicated. In previous years, our Christmas/New Year's plans have become very complicated indeed. But as this is traditionally the season for giving thanks, I do try to be thankful for these complications. Without them, I wouldn't be spending this season surrounded by people I love.