It's very common for "mommy" to be one of the later words spoken, from my experience. My DS said a lot of words before mommy (including "Daddy" and "doggie," so similar to your experience with your dog). Even after I was pretty sure he reliably could say "mommy," he didn't say it as much as other words.
I'm a SAHM, though I don't think that had anything directly to do with it.
Although they say that "ma-ma-ma-ma" is an easy and early sound to make, I feel like that was not the case for my child when it came to actually producing words. It worked in babble but not in "real" words.
Now he's almost 3 and will literally sing, "Mommy, mommy, mommy, mommy, mommy" repeatedly.
Don't stress -- it's not an affront to you at all.