Okay, first thing first. Even if Sengoku is the strongest Marine there is, he's STILL weaker than Whitebeard. Shanks and him could probably own Aojiki like a toy (well maybe not that easily but you get the idea). EVERYBODY (presumably) is weaker than the Gold Roger.
That's because Pirates > Marines. The strongest pirate crew > the strongest marine crew, and so, conversley, the weakest pirate crew > the weakest marine crew. That's why this is the Great Pirate Age. Criminals today just can't go wherever and whatever to anybody. They have to hide from the law. Pirates in OP don't have to hide from nothing. They basically cruise around plundering whatever the hell pleases them. Sometimes they get into fights with the Marines who try to stop them, but as we've seen many times, they don't have the resources to deal with it all. BUT it's not just numbers, it's power levels. Pirates are strong enough to fight and beat Marines. The ONLY reason why they haven't just overthrown the WG is simply because they generally don't want to – all they want is their freedom to plunder and go do what they please.