My opinion is this story is about control(with a lot of wonderful graphic sex of course
)so for me losing is about the demonstration that each of them thinks is in control but is only an illusion.
Dan has already lost. He gave in to his voyeur kink, thinking he was the one moving the wires. In the end his nemesis is enjoying is wife in a way he has never done and he has totally lost the control about it. In the end he will raise someone else child due to is arrogance about judging Lester poor appearance and skills.
Lester is presented as Superman. A real one leads man and Lester does not. He thinks he is very skilled in manipulating situations. In the end up to now he has only done in shadows. Now due to Sarah’s level he has begun to hit the open ground. He must learn that he has limits too and his arrogance would lead to a fail of his plans.
Sarah is enjoying sex, playing often the reluctant one, thinking to be smart enough to manipulate the others getting pleasure she has never believed to experience without consequences. The BC thing without telling anyone while teasing Lester(which is a clear indication that she has not any real or relevant feeling for him) is the evidence of it. For her losing is the lost of her control about her fertility and getting pregnant.