I have said this before, but I will say it again. Every normal person drives around in a family car. They plod from A to B and lead dreary lives, but let's say you were given a Porche or a Ferrari. Would you let others touch it, look at it from it's pipes to under the hood? If they asked to go for a spin in the car, if you knew it wasnt going to be damaged would you refuse them? It's like owning the best thing in the world, the only thing in YOUR world and people want to know, even for a moment, what it is like to be you.
The build up to the moment is everything. Watching her being striped naked by another man. Then watching as he spreads those pussy lips so he can taste what you taste all the time. Then when the moment arrives and he is positioning his cock at the entrance reserved only for you, you get second thoughts, doubt that things will never be the same again, jealousy that he shouldn't be doing that to your private sanctuary. Then something else takes hold, LUST. As he penetrates and her pussy lips part, you will find yourself holding your own cock as he pushes deeper and deeper until you hear her groans. The rhythm builds and her cries get louder as this extra spare cock is doing to her what you do, making her cum with every thrust until he stiffens. Then with a couple of extra thrusts he pulls out and you see the strings of cum stretching from the end of his cock to her pussy.
That's why we do it again and again, always trying to re-enact the first time
I hope this helps