[cloudmountain]

If the only way you could buy Aspirin was from a drug dealer, then Aspirin would be a gateway drug.

If "ifs and ands" were pots and pans there'd be no work for tinkers. "If's" are true FR fodder.

[USFRIENDINVICTORIA]

I suspect that you are not as obtuse, as you're letting on. Nevertheless, let me spell it out for you (without a single “if” or “and”):

Pot is an illegal drug in most places. The only way to buy an illegal drug, is in the black market, from a criminal drug dealer. Criminal drug dealers often also sell other “harder” drugs. Such criminal drug dealers have an interest in getting you hooked on those harder drugs. The only “gateway”, is in front of your drug dealer's house.

Where pot has been legalized, you can buy it from a law-abiding business person (just as you can buy aspirin from many legal businesses). You are not exposing yourself to a criminal, with an interest in up-selling you to a harder, addictive drug.