i hope i didn't answer this already but i am too lazy to look through pages of posts.
first off, i just have to say that love can be worse than drugs, at least for some people. i have seen people become completely irrational because of it. i know through my first couple of loves that i did more stupid shit because of love than i have ever done as a result of loss of judgment from drugs (except for ambien.) perhaps that is a bit beside the point (since drug use can be indicative of issues other than loss of judgment, eg escapism.)
i could be with a person who was currently not using drugs but who had used them in the past or would experiment with them in the future but not someone flat out opposed. one reason for that, i think, is that i can really only click with people who are open to new experiences (and ideas.) sure, drug use isn't the sole predictor of how open one is to new experiences, but it is hard for me to believe that someone who is truly open to new experiences has never/would never try drugs, if only a couple times.
a couple people on here have said something like 'people who don't do drugs aren't missing out on introspection, life experiences, etc,' but, for me, it is much more about having whatever personality factors predispose peoplet experimenting rather than the impact that experimentation has on the person.
also, i would find it very difficult be with someone who didn't believe in legalization simply because i'm extremely libertarian, but it wouldn't be an automatic deal breaker.
dating an addict is a different story. i did date one on and off for a long time, and it was like pouring gas on the fire, but many beautiful things came out of it nonetheless. it was actually a situation in which i think he was more addicted to being in love than to drugs, interestingly. in my experience, once a person is very far into addiction, the addiction usurps a person's ability to make choices that don't hurt the people they love (different point for everyone, of course), which is what makes being with an addict so fucking hard. however, i would probably do it again.