Majalis designed Goblins to only be satisfied after multiple (many, many, many) ejaculations. I know I typically play with my charisma no less than 5, so maybe that makes it easier to satisfy enemies (I have yet to test it).

You just have to keep sucking them off. Try seducing and choosing options that increase their lust whenever you can. 

I'll actually go ahead and SO this encounter and let you know how many times it took, just one second and I'll edit.

EDIT: Hmmm, well I tried and here's what I got (not including the time I hit surrender, that button rly should be placed somewhere else imo)

• First try new mage 5 charisma w/ catamite perk - Got to about 15 ejacs then I started spamming to SO, successful
• 2nd try new Warrior 5 charisma w/ catamite perk - 121 ejacs b4 she killed me
• 3rd try my lvl  5 Mage that I've gotten DA on male goblin - same problem as warrior
• 4th try my end game Hiro from several versions ago 8 charisma, catamite, mouth maniac 3, and - SO still won't trigger
• 5th try new Paladin - First Goblin encounter, says Female on the map, but actually was the male goblin with his usual intro. I tried to get SO to trigger, but I got stuck in a grapple. I'm on my back and only can rest while he keeps choosing to grapple.

I thought there has to be some kind of bug involved with the encounter but I tried one last time with a Ranger 8 Charisma, 3 Erotic, 1 Catamite, and 1 Cum Drinker (I also made them Orc-blood, fat, beestung, cute). This time it was a female goblin, but we eventually hit a point where her next move is nothing but "Do Nothing" and I can blow her endlessly but I stopped after hitting -61 stamina. So I struggle and break free. At that point did analingus a few times, reciprocate 69, got face fucked, and then got SA

So what I think may be happening is that you have to let her anal top once, then try for the SO (maybe she has to top both oral and anal before either Satisfied ending can happen). Or there is just some kind of bug going on