Still haven't done it yet, but it's on my to-do list.
What's the appeal? Well, it's the ultimate sign of sexual fertility, so maybe it appeals to the same part of the brain as big tits or shapely hips do. Though personally I'd prefer it if she's also managed to keep a good figure otherwise, as overweight women don't appeal to me.
For some, there might be a BDSM element, the fact that she has allowed herself to be "used by a man" to that extent.
And, yes, I've heard that while female sex drive normally drops during pregnancy, it has the opposite effect in some cases.