This is just my observation.
Some people can and do enjoy a "no strings" sexual relationship. This gives them enough of the nurturing physical touch and the excitement that comes with having sex, without getting mired down in all the details and stressors that can strip a relationship of energy and excitement.
Others are just not wired that way. Some need deep and meaningful connections before they can move onto the sexual component of a relationship.
Maybe it's a personality thing.