How does he not?
That's a valid point of view, though I personally don't think that the relationship of Luffy and Akainu is that significant. Luffy got over it; he remembered his friends and had as his main goal to become stronger to protect his friends. He and Ace himself accepted Ace' death, as he lived his life as a pirate. Shanks said, a man should look forward after he cried. Garp himself also holds no grudge anymore, since I don't see Garp attacking Akainu at the HQ, and that's when he wanted to kill him at MF.
So I think Luffy will comfortably let his father and Sabo tag team Akainu.