Nicotine lowers blood flow to the penis, which makes it harder to get an erection. Think of penile blood flow as water shooting out of a garden hose. Then nicotine ambles over in a garish Hawaiian shirt and absently stands on the hose, causing it to go limp.
Over time, damage done to the blood vessels by smoking can affect a man's ability to achieve or sustain an erection. In other words, although it's not 100% clear that smoking will cause impotence, the evidence out there suggests a link. By not smoking, you are probably lowering your chance of having erection problems. To my mind, that sounds like reason enough to put down the cigarettes.