That made complete sense to me. Not only were Holidays always about his mother, her wishes, her wants, her needs, but she could also put me down and never, not once, did my MEM stand by me. When I asked him why he would always take the sides of his family of origin he claimed he didn't take sides but there was some truths in what his family member was saying so he couldn't say they are wrong. It didn't matter if they were wrong, he wouldn't ever admit it. A MEM is a coward, even if he thinks he is doing the right thing to live with himself, he is certainly not doing the right thing to live happily with a spouse.
If you are dating someone who always sides with his family of origin, do yourself a favor and leave. A spouse should never come in second or third or worst.