For myself I think there's nothing strange with that, it seems that Marge really has fed her children. And story writers rally has meant it, even there's no proof of that. Still, I think I understand you with this question, what if Marge has really not fed her kids. How would she couple with it? Do she feel herself guilty and had this an influence at her present relations with her relatives?

There isn't a moment like that in the show. But Marge has said she loved her childeren plenty of times, so I don't think we need the breastfeeding thing as extra evidence.

Edit:
I have found one with Homer feeding Lisa. (not breastfeeding ofcourse)

Heh, as for sleaze - I don't wanna see parts of Marge's body I haven't seen before, and in "Large Marge" or "Natural Born Kissers" it was good that they showed nothing (and also great that in case with female characters they are even more careful; simply can't imagine Lisa or Marge in sutuations, which are so usual to Bart and Homer with they partial nakedness).

But just a hint, please! Although I can't say I believe Marge avoided this type of feeding because she cared for her body, sometimes it seems that she cares more for physical conditions of her kids than about deep emotional contacts with them, and it makes her kinda "robo-mommy", machine for raising kids (well, just a bit, but still). Yes, it's hard work, I suppose, but many mommies in Springfield look not very caring. Actually, on surface it's hard to believe that any woman in town can breast-feed her kids, if even Marge doesnt. Brandine maybe?

But I remember one moment from the show, that probably can be loosely related - I suppose, everyone here remembers that wonderful pink-coloured scene in "Lisa's first word", when happy Marge sits in a chair with baby Lisa in her arms and Bart comes to visit them. There without any hints there was so warm and kind atmosphere, that I can imagine, that it was just after Marge fed her, without any proofs  
And Marge still makes wonderful breakfasts and lunchboxes for all members of her family now. Probably I exaggerated a little about importance of this thing, I just was curious. Yep, maybe George right, it's the thing, usual enough to hide it behind the screen, but for me it still could be great to have some related moment. If they could show births, with cords of babies, I suppose they could find a way to do it not sleazy, but in a beautiful way.

tootsie
I myself always wanted to do this, but I suppose only great artist could do this in non-sleaze way Yeah, if someday I'll see this plot in a fanpic, I'll be happy