I like that Vegata/Bulma was a couple of circumstance. Vegeta needed a place to stay, everyone understands it's not a good idea to just leave him be as a normal Earthling, Bulma has a gabazillion dollars and great tech for training so he stays there. Being around together for years they hook-up but for years it's nothing romantic, even with a child Bulma knows she can take care of Trunks herself and Vegeta isn't particularly involved or interested until he finds out Future Trunks, the second person he witnessed become a Super Saiyan before himself, is his son.
By the time Buu saga comes around where Vegeta finally grapples with his identity as a warrior in conflict with the relatively domestic family life he's unintentionally developed over the years, Trunks is 8 years old.
