((Sorry; forgot to get the settings for the poll in place before I hit post, so had to delete the original post. Here's the poll for real!))
Robb Stark has wedded and bedded Margaery, and with that bedding the Tyrells and their followers are now his. But with Stannis bolstering his forces with many of those who formerly followed Renly and preparing to assault King's Landing, how does Robb react? How does he make use of the large Tyrell host, as well as a capable commander in Randyll Tarly?
Note that most if not all of these options would likely include some level of Robb/Margaery smuttiness.
Wait in Bitterbridge with the Tyrell host
There's no sense in being reckless. Robb chooses to wait at Bitterbridge with a considerable Tyrell host, and thousands more waiting in the wings at Highgarden under his new goodfather Lord Mace. Stannis is welcome to assault King's Landing, and with the Blackfish and Robb's Northern army preventing Tywin from offering any relief, the capital is likely Stannis' for the taking. But whether he takes the capital or the Lannisters hold onto it, the victor is sure to suffer losses, and that will improve Robb's future chances.
This does leave it up to the Blackfish and the Northmen to keep Tywin in check, and there's no guarantee that they won't suffer significant loss. It's also possible some of the Tyrell men might be impatient to enter the fighting after sitting the war out under Renly and then having him killed. His mother is also worried about Sansa and Arya being trapped in King's Landing when Stannis makes his assault. Robb is worried for his sisters as well, but they will be in danger no matter which army takes the fighting to King's Landing. Robb will follow Renly's example, exercise patience and allow his enemies to weaken each other.
Ride for King's Landing immediately
Robb set out from Winterfell intending to march on King's Landing, and now he finally has the opportunity to do so. He will waste no more time, not even to wait for the Blackfish and Theon to bring the Northern army to join them; he and Randyll will take the Tyrell army to King's Landing with haste. It's highly likely that his path will cross with Stannis either before or even during the assault on the capital since the sole remaining Baratheon brother has his eyes on the throne as surely as Robb does, but that is of little consequence. If anyone is going to take King's Landing and see to it that Joffrey pays for his crimes, it's the son of Lord Eddard Stark, and he'll do so with all of the strength currently with him.
Send Randyll to help with Tywin while dealing with King's Landing himself
With the size of his army now that the Tyrells are supporting him, there's no reason Robb can't look in two directions at once. He'll place a sizeable chunk of the Tyrell host under Randyll's command and send them West to bolster his Northern army in battling Tywin. The rest will ride with Robb as he marches on King's Landing and attempts to take the throne that he pledged to take when he took Margaery into his bed, and equally importantly for Robb personally, attempts to rescue his sisters as well.
Crush Lord Tywin and the Lannister host
Stannis may have his eye on Joffrey's throne, but it's Tywin Lannister who is the real threat. The Blackfish and the Northmen are harrying the Lannisters, but if Robb brings the might of the Tyrells into the fight as well, they can do much more. Randyll can take a significant portion of the host into the Westerlands to pillage and raid, and do enough damage to Lord Tywin's lands that he'll have no choice but to respond. Once he does, Robb, the rest of the host as well as his Northmen can spring the trap and move in for the kill. If the newly combined might of the North and the Reach can slay the lion, they can then turn their full attention to King's Landing and march on whoever happens to hold it by then.
Focus on Stannis first
Robb wants to take King's Landing and hopefully end this war as he does, but in order to be able to make his march on Joffrey's seat, he first needs to ensure that Stannis and his army will not interfere. Since peace or alliance will never be possible between them, Robb must crush brother of the man he had been named in honor of. Fortunately for him, Renly's old seat of Storm's End has not yielded to Stannis in the brief time it took for the Stark-Tyrell union to come together. While Stannis lingers outside of his house's traditional seat, Robb can strike him quickly and with massive force.