As I recall from the novelization and the character notes in The Making of ST:TMP
, the idea was that Decker was strongly drawn to the kind of unitive, spiritual experience that Deltan intimacy offered (the novel says he was drawn to the "New Human" group-consciousness movement), but wasn't ready to make that kind of commitment because of his career ambitions. So it's not like it was some ugly secret that was sprung on him and caused him to run away to "protect" himself. It's something he wanted but was afraid to embrace fully. So essentially, yes, it was almost exactly like Riker and Troi -- well, the unity wouldn't have been as deep, but Riker also pulled away from commitment because his career ambitions came first.
But by the end of TMP, Decker's career ambitions had been thwarted by Kirk, and maybe seeing Kirk's driving ambition to command made him question his own ambition. So that careerism no longer overrode his craving for something more spiritual. A merger with V'Ger offered him the kind of transcendent, consciousness-merging experience that he'd long been drawn to, as well as being his one chance to be with Ilia.
Unfortunately, most of that side of Decker's character development was cut out of the movie, so his motivations were left rather unclear.