The hug should have been expected, but Seamus had managed to convince himself that that stage of their relationship was behind them and he didn't quite know what to do. His natural reaction was to melt into the contact. The time since Dean's birthday was probably longer than Seamus had gone without a hug in years, and he didn't like it, wanted to recapture that human contact. On the other hand, his skin crawled at the idea that Dean was hating this, suffering through it despite his natural aversion.
In the end, conflicted, Seamus leaned into the hug with the unscarred side of his body, wrapped that arm around Dean's waist and tucked his head into Dean's shoulder for a moment. "You and me both," he muttered. "Are you getting jam on me?" He pulled away, as if to check himself for sticky smudges but mostly because he didn't want to prolong the contact past the point at which it was comfortable.