My answer is that the world of dungeons and dragons is a fictional world, and situations like the one you mentioned is a perfect example of something that should be left to the GM's discretion. Do they or don't they want pretty necromancers in their world? Does the magic unnaturally rot a lich's flesh away or is it just accelerating the rate at which they decompose, at which point a Houri would indeed not rot? Some people may like the idea of an ageless lich, while others will think it's a "twilight" take on a lich, trying to make something that is suppose to be hideous and scary pretty and alluring.