The season 4 finale of The Handmaid's Tale plainly spells out some plot developments, but it also implicitly supports the theory that Serena Waterford and Mark Tuello are having, or have had, an affair in the past. A good chunk of the episode focuses on how the Waterfords' odds are increasingly shifting in their favor. Despite being pariahs from Gilead now that the nation knows about Fred's involvement with the Canadian government, it looks as though they might be able to secure a fairly ideal future for themselves within their new lives.
As many victims feel when justice seems to repeatedly fail them, June begins to wonder if she and her fellow ex-Handmaids are the only ones who will have to carry the weight of the horrors Fred Waterford perpetrated in Gilead. She starts preparing herself for the possibility that the Waterfords will have the stereotypical family unit that they've always wanted. After all that Fred has done, it even looks like Serena is willing to slowly retry their relationship; she'll do anything to have a husband and father figure for her unborn son.
She really leans into this decision when she speaks with Mark Tuello outside of the meeting room while Fred is still answering questions. And, on the surface, it does seem as though the two are merely talking about Fred's treatment by the staff during his cooperation and what the future might look like for the couple now that he's an intelligence asset. But their interaction on this episode of The Handmaid's Tale is also an incredibly loaded one; it feels far more personal than professional. And things really kick up a notch when Tuello asks Serena if she plans on living with Fred during their new life. She says they'll stay together as a family, but it's clear that what she's saying heavily weighs on her. Tuello replies with what seems to be the scene's most telling line: "Can you explain that to me, Serena?
He says it with such a sadness in his eyes; it's almost as though, despite the heinous crimes the Waterfords have both committed, Tuello feels sympathy for Serena. He looks at this woman who has lost her finger, who was treated as an inferior being back in Gilead, and he sees that she's out of options. Granted, this could simply be a moment of human compassion, but he seems to care about her more than that. It certainly supports the Handmaid's Tale theory that the two are either having, or have had (it would be harder to keep under wraps now), an affair of some sort. Ever since Tuello first made contact with Serena, there's always been a certain chemistry in the air between them.
And the way Serena looks at him after he asks that last question speaks volumes. She says she doesn't have to explain herself to him. But, with melancholic instrumental music in the background, she looks as if she could almost start crying, and quickly breaks away and goes back into the room. Though it's highly doubtful that Mark is her baby's father (a rug-pull would've happened by now), there seems to be more to Mark and Serena's dynamic on The Handmaid's Tale than sorting out legal/administrative needs and arranging a possible, new life for the Waterfords.