An opposed landing puts the attacking force at a severe disadvantage. Since the seaborne force is generally more mobile than the land force, they could just sail to another beach and disembark there. There are plenty of non-opposed landings...Troy, Marathon, etc.

Caesar's first landing in Britannia was opposed by the Britons, however. The Briton's cavalry and chariots were able to keep pace as he tried to find a better landing beach. Eventually he probably came ashore near Walmer, in Kent. The ballistae from the ships were able to keep the tribesmen back from the beach, but the Roman legionaries balked at jumping into the deep water. It was not until the unnamed aquilifer (eagle bearer) of Legio X lept off the ship crying, "Leap, fellow soldiers, unless you wish to betray your eagle to the enemy. I, for my part, will perform my duty to the republic and to my general." (At least, that's what Caesar claims). He did make it ashore, but his ships were wrecked in a storm, and he was unable to get resupplied (having left his baggage train in Gaul). The Romans were eventually forced to retreat back to Gaul.

Gaius Julius Caesar, Commentaries on the Gallic War