I have had a lot of friends throughout the years who smoke weed for various reasons. While I don't think it makes the person a bad human being, it indicates several things that would make them an unsuitable mate for me.
1. Usually weed is a way to numb out feelings that are uncomfortable to deal with or to avoid dealing with a situation that is not easy. Not only are numb feelings inconducive to intimacy, but I think when you start finding ways of just coping with problems in a temporary way, that often spills over into other areas of a person's life.
2. Weed often attracts friends that are just looking for someone else to smoke with. While smoking weed in and of itself doesn't make for a bad friend, bad friends often are chronic weed smokers. These aren't the people I want as my main social circle.
3. The fact that weed is illegal and the person is doing it gives me some pause. It is the principle of it for me. Once one thing becomes a decision each time, lots of other instances of following the law can become time by time decisions, rather than foregone conclusions. I don't care for the attitude behind that or the risk that it could inadvertantly put on me.
4. Weed smokers tend to underestimate the impact that it has on their lives in a number of areas. Smoking unfiltered cigarettes of any kind has long term health effects, motivation isn't something that usually drops in one fell swoop, negative coping mechanisms and habits tend to develop over time rather than all at once, many people don't understand that it impairs their driving and so are putting others at risk and smoking weed regularly has a significant economic impact (much like smoking cigarettes). It is also an example I don't want to give my children or the other people who are involved in my life who could be influenced by it. Just because some people manage just fine doesn't mean that others will. Most serious drug users that I have been friends with or close to started by smoking weed and it was the friends they were with through that which contributed to them adopting a lifestyle with more serious drugs.
5. Because it isn't something that I'm interested in doing, it would drive a wedge between us. Studies say that even eating different menus regularly than the person you live with has a distancing effect. How much more something like this, which also has some sense of subculture that often accompanies it?
6. Because marijuana isn't a controlled substance in North America with specific guidelines, it is very hard to know for sure what may be in the weed you are getting. This can prevent a very serious health risk. I have had three people close to me rushed to the hospital with rapid heart palpitations over weed cut with other substances. In some cases, even the people dealing don't really know what is in what they are selling. I don't like the thought of losing someone I care about to something that needlessly.
7. The money going to buy weed supports people that I don't want to support. If my SO was regularly smoking pot, I would have an ethical problem with where the money was going to and how it was affecting many others in our country and beyond.
8. Longterm health effects are not clearly proven with the present levels of THC. While some warnings can be dismissed as anti-drug propaganda, there are others that I feel uncertain about. I don't want to be with someone who doesn't practice self-care (not doing something that would put themselves in needless, preventable danger).