The s/m concept of “vanilla” sex is sex devoid of passion. They are saying that there can be no passion without unequal power. That feels very sad and lonely to me, and destructive. The linkage of passion to dominance/subordination is the prototype of the heterosexual image of male-female relationships, one which justifies pornography. Women are supposed to love being brutalized. This is also the prototypical justification of all relationships of oppression — that the subordinate one who is “different” enjoys the inferior position.