I was under impression that when combat can end in dominating the enemy, this is achieved by fucking them several times. So far, I have managed zero domination victories, because I tend to give up after nothing happening in a dozen fucks. Is there some additional condition? Do I have to do them in a rapid order? Does it require a special sequence? Do the words 'You cum" or 'Your opponent cums" in the log box even mean anything? They don't seem to impact erections or lust.

Have I somehow broken the game and that is causing all of this?

Check the endings widget on the right side of the screen. Not every character has a domination ending.

I've been wondering this as well, specifically for the goblin. It is clearly marked with the domination victory as possible, but I have fucked them maybe 20 times over in multiple positions in a single fight and still not gotten the domination victory. All the other times I have gotten it after only once or twice.

It is possible, I've gotten it plenty of times, it just takes a lot of tiring them out, wrestling and such until you satisfy them, I don't know the parameters for it though.