I don't think sex itself is mandatory but if two people are having sex then it is mandatory that it is satisfying for both. I am of the variety of people that believes if you can't get the sex right then a committed relationship is a waste of time. I'm talking about proper intimacy by the way, not just bumping uglies. It is the ultimate symbol of trust, communication and respect (things which I think ARE mandatory), but it is not the only one.
I don't understand what all these percentages mean by the way. Sex is not mutually exclusive to any other method of expression. Defining such things in absolute values is misleading anyway.
