What it took to get there was far from tender. Virginia has been trying to work her way back into Bill's heart and arms all season. She realizes how much she hurt him - she of all people would know that - but couldn't seem to find the right combination of circumstances to help this reunion happen.
In the end, it took the two of them going under cover as a couple who needed sex therapy, a very convincing guise seeing as how they needed just that in their real lives. Even then, Bill didn't want to perform with Virginia as part of the ruse. She had to say "it's only sex" to Bill, to get him to not only have sex with her, but make love to her, too.
The unlocking of emotions at this tender moment was beautifully played. The therapists on the other side of the glass, of course, had no idea what they were seeing. They congratulate themselves on providing the circumstances for their patients - Bill and Virginia in disguise - to have had a successful sexual experience, but they have no idea what they're really seeing.
We the audience do. Bill and Virginia belong together. They have every reason to be together, in all ways, as Virginia has come to realize this season, and as Bill may now be finally beginning to realize, too.
The course of true love never did run smooth, especially when it comes to pathbreaking researchers into the libido, who have been from the beginning their own very best subjects.