Depends. In Goku's case, it was a vehicle to have kids for the next season. Personally I think it's a small miracle he knew what sex was (and not to mention the old Man of Steel, woman of Kleenex bit.). It only works as a comedy where the punchline is "he's been roped into this and has no idea how or why). If anyone suggested otherwise (like there was genuine love) I would laugh at them.
In the case of One Piece, Luffy has been shown to be attracted to all of one woman in Nami and Oda apparently retconned that in the dumbest way possible. If he ended up with Boa, unless it's another "she tricked him but he has no sexual drive, LOL" it would be beyond ludicrous.
I guess because they're shown as asexual and not caring it works as that one joke, but if you're going to convince me they're serious, especially in Oda's case where he retcons anything that suggests Luffy is anything but asexual (like Strong World where it feels all but stated in the message Luffy doesn't hear that Nami loves him.) You do not put all this effort into ensuring he's asexual and then suddenly give him a libido.