Makes sense that that happens because once THC kicks in, it's a powerful vaso-dilator (vein relaxer/widener), which is the mechanism that erections depend on. Those veins have to relax and open up so the blood can flow into the erectile tissue. That's why people with high blood pressure have the opposite problem of having difficulty getting hard. Because those veins can't relax when they're under so much pressure.

You may have to quit smoking except in private. Or wait to smoke again till you're a little older and the response you're experiencing now isn't so automatic.