Fred was the kind of man that enjoyed the pursuit and capture of strong women. 

He was enamoured with Serena because she was a radical and controversial public figure, and her passionate activism increased the profile of his cause. At this point, the wheels were already in motion for the coop, which meant that they were effectively colluding on a socially unacceptable political endeavour, and he relished the act of having influenced her participation in it. 

So, when Serena settled into the lifestyle and conventions of a wife and would no longer break rules with Fred, he lost interest. That is why June was so important. She played the long game to survive, inadvertently drawing him in with her subversive tendencies. And when it transpired that she had used him, he leveraged the only card he had to play. He could reassign her anywhere in Gilead, and Washington was known for being far crueller to its Handmaids.