@IgnoranceNotOk
I completely disagree, and I think you're remembering childhood Christmases with fondness and loveliness, because that's how you perceived it as a child! What about your mother and the other women in the family who had to do all the grunt work and all the peeling and the chopping and the cooking and the baking and all the washing up and cleaning up after, while the men fucked off down to the pub?! (Or sat in front of the TV!) Coz that is what happens in most families.
It's all very well saying 'wouldn't it be great to have 15-20 people for a big, massive Christmas dinner and 'fun party?' But the fact is, somebody has to do all the work to produce this wonderful Hallmark Christmas movies type of scenario. And yes, it's nearly always the bloody women doing it? (And someone has to pay for it too!) Dinner for 17 anyone.... ?! Who is paying for it?! Everyone coming gonna chip in? Yeah right, like THAT will happen! You are also assuming EVERYONE will get on!
You have a very romanticised and airy-fairy version of great, big extended family Christmases in your head, because it's what you remember as a child. (Even though they may not have been like this, and were very likely just people popping in...)
If you were having to host - and pay for - and provide for 15-20 family members all over Christmas Eve, Christmas Day, and Boxing Day, you wouldn't think it was so amazing.
I know a couple of people who had 10 to 13 people staying, over the last three or four days and they've been driven absolutely batshit with stress. They waved goodbye to them all today and both of them said they could not wait to see the back of them. It's been the most stressful 3 or 4 days of their life.
As a previous poster said, it's also extremely offensive to say that you can't have fun, and a lovely happy joyful Christmas if you are a childfree couple - or have one child or two children, and it's just you/your DH, and the kids. I find it quite depressing that a few posters on here have said 'it was just me and DH and the kids and it was really dull and boring!'
WTAF?! Do people really have such a serious lack of imagination, and such a boring, tedious life that they can't think of anything to do with their partner and their kids over Christmas? 
When my kids were little (they're in their 20s now,) we used to go to Christmas tree farms, we'd go and see Santa, we'd go a nativity, to a panto, to Christmas lights switch-ons, to Christingle, and to Carol services. We would go driving around looking all the Christmas lights in all the towns. We go to the indoor shopping centres, we'd make Christmas cards, we'd play games, we'd watch multiple Christmas movies, we'd go to the cinema, and we'd spent a couple of days decorating all around the house and decorating the tree, we'd go for lovely walks, and we would bake cakes and do crafting (making Christmas stuff.) etc etc etc...
Now our kids are grown and left, me and my husband do lots of the same stuff. All the fun stuff. We're nearly 60 now and we're like a couple of five year olds. Ran around a local Christmas tree farm for 2 hours the other week, went to a panto, went to a theatre show in Birmingham, played Monopoly, and several card games, and scrabble, made some Christmas cards, went to a nativity, went to 2 Christmas Church services, and went to 3 Christmas lights switch-ons. (And many other fun things!) We have the best fun together. Just the two of us.
Yes, we still celebrate with our adult children and their spouses a bit; we go to a show with them, or out for a nice meal around 18th December, and we see them on Christmas day, and have our Christmas dinner together, or a buffet, (and play a couple of games with them too... like Cards Against Humanity.) But for 90% of Christmas it is just me and DH. And I absolutely fucking love it. So does he.
The big hallmark movie family Christmas that you're romanticising about is just a fantasy for probably 97% of people in reality. Most people don't have it - and the ones that do find it really stressful and really expensive... You only have to trawl through the AIBU, CHAT, and RELATIONSHIPS boards here to see a myriad of threads featuring stressed and frazzled women who have had a shit Christmas because of extended family and their demands... The Waltons family Christmas is basically a fantasy for most.
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