Metaphor for getting a boner under what is usually a blanket, but can also be a jacket or shirt (as long as it's thin enough so that the boner still shows). This phrase comes from the boner acting as the mast that holds up tents (blanket or sheet, for instance).
Morning wood is a specific situation when raising tents occur.
In the darkness of the room, with nothing but the flashing light of the screen, Alex felt himself getting a boner from the hot intimate scene. "Look, guys! He's raising tents!" Angela said as she pointed at Alex's clearly growing lump between his legs.