it's actually a complex matrix of things that go on that lead to this, but all basically step from the patriachy! 
So, there are societal expectations for women to take on the social/caring responsibilities. This is then exacerbated because quite often, because men are NOT expected to do this, when they do, they do it very badly eg they invite you over for Sunday lunch and he agrees. But... he doesn't tell you so on the Sunday as you're getting ready to meet your girlfriends for brunch he anounces that you're supposed to be at his parents. Now yo're pissed because either you have to let your friends down, or you get guilted into going to his family and looking like the bad guy if you don't.
At which point, and this is crucial, it gets turned on you..."Oh, well you should handle all the social plans because I'm not good at it" (he might even say these words, but more likely it's a subtle decision making process). And of course, it's infuriating because even if, for example, you text his family to say you're sorry you can't make it because of prior arrangements and their darling son forgot to mention this to you, the response will be "haha, what is he like. It would be so much better if she just handles it - so I'll call her next time".
And so the pattern continues to repeat and somehow, not only are you doing it, but you're STILL getting into trouble. Because you will mention to him that his parents want you to come over for a birthday bash and he'll say something like, "oh, but I think Bill and I are playing golf that weekend. I'll talk to him/" And he doesn't. And you're left hanging and you can't reply to his parents.... who, of course, blame you.
or you'll agree to invite his parents over. But he doesn't realyl want them over. So you're running around like a headless chicken cooking and cleaning and when they come in, they're judging YOU because the house isn't spotless ... and feeling sorry for him.
It's totaly shit.
Start as you mean to go on - you're goign to find yourself in shit anyway so just say "Please contact Dp about plans. Hope to see you soon" and leave it at that.