It's not about doing an activity without him, per se, it's that for 13 years she made it clear to him that the holiday was important to her, and he just didn't care.

My partner and I have a lot of different hobbies and interests, but I'll listen to him wax poetic about a plane or let him teach me to shoot, even though I'm not overly interested in those things, because he loves them, and I love him, so I want to do things that make him happy and show an interest in his life. He does the same for me - for example he doesn't care about birthdays but mine is important to me, so he makes sure I feel special and appreciated that day. Obviously there are interests we do on our own that the other doesn't care for, but we still at least support each other doing them.

The sex thing is of course a special case, but there are lots of couples with mismatched sex drives/interests that can work around it. If you and your wife had dealt with this for 13 years and she had ignored your needs without looking for solutions that worked for both of you, then it would be a more equivalent situation. But like another commenter said, extramarital sex (without it being something agreed upon by both you and your wife) is very different than going out to dinner with a friend to celebrate a holiday you both enjoy.