Mitchell and Curt are a couple whom Will went out with separately.
Mitchell was Will's rebound guy after his break-up with Michael some years before the start of the series. They went out on one date and "fooled around" in Mitchell's apartment. Will recalls that because there was no heat in the apartment, Mitchell asked him to "cuddle me bum", hence Will has since then referred to him as "Cuddlebum" in retelling the story. Mitchell thinks Will has clammy hands and in turn refers to him as "Clammy Hands". Shortly after Will, Mitchell met Curt.
Will met Curt at the gym years after his fling with Mitchell, and right after Curt had just broken up with Mitchell. After one date where Will apparently "nibbled" on his neck, Curt starts avoiding him by changing his workout schedule and not returning his calls. When Will eventually finds and confronts him, he admits that Will is his rebound guy.
After making amends with Mitchell, Will later finds out that Mitchell and Curt are actually a couple and had gotten back together after his fling with Curt.