Not sure the author really thought about that biological advantage, I mean being unable to produce your own females is a huge biological disadvantage, you're entirely dependent on other species that you go into direct conflict with.
Not if it where female only goblins, that would be a different matter.
I think that makes even less sense.
You and the other guy are making the assumption a female goblin would be attractive to human men. If we're talking about an evolutionary standpoint, they would have to almost exclusively rely on breeding with humans or humanoids to reach the point where human men would want to fuck them, since what a human might find attractive isn't the same ass what a horse or a dog finds attractive.
On the other hand, being a male that can breed with
anything means that you're theoretically not just limited to humanoids. You don't need to be attractive to a female to rape and impregnate them. Also, as a male, you don't need to wait X amount of days before you can make more kids. As a male, you aren't burdened with with the needs of pregnancy and potential imbreeding is less of an issue versus one dude siring multiple generations of goblin females.
But honestly, the debate itself is somewhat in bad faith because you're comparing two different genres. If goblins were all-female, ugly as shit, violently murderous, and whatever else you'd associate with a male goblin with the only difference being that they have a slit instead of a dick, you wouldn't want to fuck them. If you want to argue male vs female viability, you'd have to argue about them in the same metric.
Like if male goblins were irresistibly sexy fucking studs and were treated like female goblins, I 100% guarantee you that being male-only is more evolutionary advantageous due to the fact that a single goblin could sire kids with as many women as he's physically able. With a female goblin, you're bottlenecked by the gestation period and however many ova can be released. Like even if she can give birth to 10 kids in 10 days, all it would take for a male goblin to match that is breed 1 female a day. And that's assuming no fatal pregnancy-related complications. Males wouldn't have to worry about such things.