I'm a little confused. How are you gauging the "listing"? Is the hull
not level or is just the mast leaning to port? (Ex-Cal-did you mean straight as opposed to "bent" or straight as opposed to "plumb") It could be as simple as the standing rigging
out of tune. The port shrouds might be tighter than the starboard shrouds pulling the mast to port. To expand on Ex-Cals advice, use either the main halyard or the topping lift
as a measuring tape and measure from the mast head
to a spot equidistant back from the pointy end on each side at the toerail. Use maybe a chainplate as a tick point. These distances (the hypotenuse) from the masthead should be equal.
If they are not, loosen up all the shrouds and then start tightening again with the upper shrouds first. The masthead is the apex of the triangle. Tension is important! Too loose will create slop and the banging under load will transfer to the hull
thru the chainplates. Too tight will create stress on the chainplate/hull joint with the excess constant
pressure. Sail-loads are not constant and are calculated on rigging
during design, but the combination of the two can be bad. Borrow a "loos" gauge or just talk to an old salt
with a good ear and hand for rigging. It's not rocket science.
If this is not the case I've just wasted 5 minutes at the keyboard!