When you start a relationship with them (closed relationship), you agree to be with them, and only them. You might really love strawberry ice cream, but you might also love vainlla, chocolate, orange. The list goes on and on. The difference is that relationships are not the same as the sense of taste. A compelte devotion, and trust are required, and the attraction part, the ice cream flavor part, is only skin deep. Lust is a skin deep emotion, and one that does not need to be the defining feature in your relationship.