DISCLAIMER : NOT IN ANY CASE, I SAY WHO SHOULD LEAD OR FOLLOW, IT IS NOT OF MY BUSINESS, AND FRANKLY I DON’T CARE !!!!!
Please give me a second to explain, for people who don’t know, what sexual dimorphism is. Sexual dimorphism is a quality of a species (plant, animal…) who have difference (behavioral, physicalal) between two subgroup named (male and female). The human is a sexual domorphic species 
First obvious difference, one group make the babies, the other do not… It is pretty obvious.
But asside from this, there is some other major difference : height, weight, strength, position of the center of balance, flexibility and dexterity.
In this first installement, we will see how those traits apply to different dance role.
How does those traits relate to partner dancing ?
Let’s clear up the obvious, one gender is taller, heavier, stronger, have a higher center of balance, less desterity and less flexibility. The other are shorter, lighter, have a lower center of balance, more dexterity and more flexibility.
First, being taller than your partner, make you able to see over his body. This allow you to see that a collision is imminent, and if you add that to the responsability of leading, you can gracefully lead your partner out of the way of a collision, and injury.
Seeing the danger coming is one thing, it is not suffisant, you need to be able to move your partner out of the way, if the one doing the leading is heavier, and the other lighter. Newton law of the conservation of energy states, one object who is heavier, have more energy and more momentum at his disposal, one gender is heavier.
If you’re adding that trait with strength, you have a partner with more momentum and more force he can apply to stop a movement, and another who have less momentum, and less force to stop a movement.
If a collision occurs, their is another theorem who is usefull F=MA. The force(F) of a collision will be dictated by the weight or mass (M) times its speed or acceleration(A). If your partner can remove some of that speed, with his momentum and strength, the resulting force will be lesser. And if the mass of the two object colliding is lesser, then the chance of injury will be lower.
So one of the responsability of that partner is basically protection. That doesn’t mean that collision does not occur, it only mean, it occurs less.
In the modern version of swing dancing, or dancing, use enhance range of motion, it take the dance in the third dimension, with aerial and acrobatics. Having a higher momentum partner, help to keep those things safe, having a stronger and heavier partner who stay with his foot firmly on the ground and his ready to use his force to slow your acceleration against the force of gravity is paramount for practice and to safe execution.
Now, center of gravity, having a higher center of gravity, make the turns and I mean turns more unstable, and having a lower center of gravity, make it more stable. If an object motion in unstable, you must have the hability to correct that axis of a spin, by shifting your center of mass, so it align again with the axis, or correct the axis of rotation.
Being more flexible, more precisely near the center of mass, one gendre who incidentally carry the offspring around the same center of mass, has an advantage over the other, who carry his upper body strengh way higher, on his shoulder, and therefore have a center of mass much higher.
Having flexibility near the center of mass, around the hips, make it easier to micro correct any changes of the center of mass needed, the center of mass of one partner is at the navel. We all know that most of the other gender, doesn’t have the same flexibility and in some specimen, we could assume that the flexibility is limited to only one axis of movement.
Last but not the least, dexterity, In all the good dancers, the little move, added, that give flavor to the dance, require dexterity, exactitude in the fine motor control. While one partner is seeing at the safety and enhancing the movement of the other partner, the other partner, can with his fine motor skills, create flavor, for both the partners.
This enhance the dance, the follow is basically the star of the show, not all the time but most of the time, and the lead is her bodyguard, off course, not all the times, but most of the time.
In the next installement, we will try to see how those roles evolved. I say try, because there is little research in the subject.