I sometimes wonder if people here realise the implications of Garp betraying the marines for Ace, especially after Sengoku revealed he's Roger's son. The hero of the Marines would demoralise ALL marines, besides maybe Akainu and cause havoc all across the world.
He clearly wanted Ace to become an "exception" just like him, if he's allowed in the marines while publicly calling himself a D. Ace likely would have been accepted if he rose into power and gained enough fame that Him being Roger's son would only be seen as him "redeeming" his bloodline.