I used the same type of approach, basically said, if you can be there, we would love it, but if you can't make it we totally understand. My MOH and I got into it before I even got engaged (I've always wanted a DW), and she game me the same 'you're being selfish' guilt trip. Turned out that she feels a wedding is about sharing your love with all your family and friends and I think a wedding is about devoting yourself to one person, and is only about the bride and groom. If ppl want to come see me get married, GREAT! If not, no skin off my ass, I would be just as thrilled if it were just him and I and the sound of the ocean! People have different perceptions, but the only one that matters for YOUR wedding is yours. And the grooms. A little. lol, c'mon, who are we kidding? A wedding is the brides day!
My MIL-to-be called earlier this week and went on at length about how she'd found a deal for $500 less than what she'd paid. Made my fiance feel horribly guilty. Turns out the package was the last seat left on a plane that was set for different dates and they'd miss the wedding entirely. I was furious. Why is she still even looking at prices? Just to rub our noses in a better deal? It could have just as easily went up! Erg.
Well, there is my ranting contribution. The moral of my story is just to let it roll off your back, this day is yours. Own it. Anyone sending negative energy toward it/you, don't deserve the energy you'll give back stressing about it.