And fisting is extreme. There is a real danger of real damage, especially with anal. The vagina has some elasticity to it. (Giving birth and all.) The rectal cavity however really doesn't. I remember hearing on Loveline Doctor Drew said one of the leading prosthesis are sphincters for those who mess theirs up with anal sex. (Probably one of the reasons I'm so damn delicate on the idea.)
This is just me, but understanding of my physiology and female physiology in general MOST of the sexual stimulation happens right around the lips and clit. (It's why eating pussy is so much fun for both parties.) So penetration is NOT a huge thing for me, and I don't feel any great need to be "filled" a couple fingers tend to be plenty.
BUT there are the more clitoral climaxers and there are those women who NEED penetration. So I would imagine being "filled" might be nice.
That said I still think "stretched" is another matter.
But that's just my A$.02.