That D's are supposed to be the Gods natural enemies, is something that's only told amongst the Celestial Dragons and those really do not have any kind of contact with the common people. The latter don't have any particular opinion about D's if they already would even know that there are D's all across the world. Even among the Marines only the higher-ups, like everything from admiral and above, know about the potential danger of D's, but for the rest I doubt many think about it much. If Garp entered the Marines in a recruiting office somewhere in a remote corner of East Blue, there is really no one there that's going to say "You're a D, get lost". For them he's just another soldier.
Also what makes D's D's is not that they are a unified army, but that they are a group of people who seem the share the same mentality, that they will stand for their own personal believes. It doesn't matter whether they are marines, pirates or revolutionaries. I mean Garp has never showed any intention of ever betraying or questioning the Marines, he however on occasion decided to go against his own superiors if he thought it wasn't right. I think Dragon formulated this mentality quite well: "Question the world". This is a really dangerous notion as it's the opposite of the Marines and the WG who expect you to obey any command without hesitation and to accept the world as it is without questions (in particular the position of the Celestial Dragons).
I mean in the end it was a D who was able to keep the Pirate King, a D likewise, in check. It's inherent to their own mentality that no one in particular is the enemy, so they can be as much a pirate as a marine or something else. So a D who supports them, can be quite beneficial, especially because D's themselves have no clue about what that D means, so they will not see the Marines as their enemy just because they're D's. They don't even see themselves as a clearly identifiable group. So far whenever one D met another, they didn't even care about that initial. So for them it's not really important.