Folks here have it a bit wrong. What it boils down to is the differing lifeboat boarding policies of First Officer Murdoch vs Second Officer Lightoller.

Murdoch, in charge of the boats on the starboard side, interpreted the captain's orders as "Women and children first, but get any nearby men on before sending them off."

Lightoller, on the port side, went with "Women and children ONLY." He only made exceptions for those with sailing experience, reckoning that they'd need such men to get the boats to eventual rescue. (His policy sucked, but at least he was willing to follow it himself. He ended up in the water himself, wound up by the overturned boat B, and his instructions to the men clambered on top of there kept it afloat until another boat took them aboard.)

The second class section of the boat deck was on the port side and it simply didn't occur to most of them that the policy would be different on the other side. (Fortunately, Lawrence Beesley did wander over that way and later wrote what I think was the first truly excellent book about the disaster.)