First of all, don't feel guilty either way. There is no right or wrong, black and white answer to this. You just have to decide what you really WANT!!! A couple of things to weigh in your mind (keep in mind that I have a 4 1/2 yr old and a 2 yr old myself).
Like others have said, first think of what things would be different if you went alone as a couple or with your 2 yr old. As a couple you could stay out late at night riding rides. Eat out wherever you want without having a schedule. One big thing is not having to take a stroller, diaper bag, etc. You could stay on a flexible schedule and go on YOUR timing. If that sounds like a dream world and is something you want to do at Disney in the next 16 yrs then I say GO FOR IT NOW WHILE YOU HAVE A CHANCE!!!
After thinking that through, then picture being at WDW with your 2 yr old. I do not agree with what other posters have said about not taking little kids. Although my 2 yr old won't remember the trip we took in Feb when she was 18 months, I CERTAINLY will. Both of our girls were ANGELS the whole trip and we never had even one breakdown. Even the 18 month old seemed to know that this was a special time. They both fell asleep in strollers when they were tired and we got to do some "adult" stuff while they were asleep. A 2 yr old will certainly enjoy some things but it will definitely limit that amount of things that you can do and rides you can ride. My parents went with us in Feb. and I think that was the best of both worlds because we got to go out one night and they watched the kids and then they went out Valentine's night just the two of them as well. We also then were able to baby swap and still have someone to ride rides with. If your mom would want to or could go with you, like others have said, that might be a good option - she could watch your 2 yr old as much as you wanted her to (or as little) but you wouldn't go days on end missing your child either which I think would be the main problem.
One last note, if you REALLY want a romantic trip with DH at WDW in the next decade or so NOW IS THE TIME TO TAKE IT....not after you have already taken them. We have always talked about going to WDW/
Disney Cruise for our 10th anniverary in 2009 and now since our 4 yr old will remember it, we will either have to take them along or save a bunch extra and take an extra trip with them as she would DIE if we went without her!
If you need anything else, just ask! I'm sure you'll make the right decision...don't stress over it...you get to make a wonderful choice either way that many wish they had to make!!!!