The Tower can be likened to a volcano. A volcano is one of the most violent forces on Earth. It creates and destroys landscapes, affect the weather and they can even change the composition of the air. Although the inhabitants of Santorini might feel them to be slight inconveniences, they are merely the products of the blind forces affecting them. More than anything else, they remind us that below us move titanic forces that do as they will without even noticing up at all.
And so is the Tower. It basically relates to forces that have hitherto been contained until the pressures build too much... and then there is no containing them. I think people see this card as bad is because of our natural propensity to seek out stability, and change doesn't always agree with us.
But it need'nt be bad, and in fact it can often be predicted. The Tower is secrets that are kept for too long and they pop, a roof that has seen too many winters and cracked, or in the dramatic confession of a lover. The feelings related to it are usually powerful and intense, but not always anger.
The Tower is also the most male card in the deck, being attributed to Mars, and it may signify very active energies, bursts of energy, a sense of purpose and industry.
And yes, the sexual act is a good metaphor in this case, pressures mounting until there is a violent explosion.
