Marriage is like cycling.
In the first stage of marriage, the two are riding the same bike. Often one rides the handlebars while the other pedals. The dependent partner goes along while the dominant partner supplies the power.
In the second stage of marriage, the pedaler drops the rider, or the rider kicks the pedaler off, but they both compete for the same bike, for the right to steer direction, for the control of speed and brakes.
In the third stage, separate bikes allow them to differentiate. Now they ride side-by-side, regulating their speed, choosing their common direction. They ride separately together. Each supplies her own power, each keeps his own bike going. Sometimes in tandem, one may ride ahead, or lag behind, but each keeps the other in sight or in mind.
In the fourth stage of marriage, they may be ready for a bicycle-built-for-two. Then both can supply equal power, or one can rest without being left behind, but they ride individually, together.