Instead of asking them to join you right away, ask them what they think about using recreational marijuana. Asking them about their opinion from a neutral standpoint (a simple "I noticed its legal in this state, would you ever smoke?" etc.) will provide an insight on how they will react to your next question.
By all means, ask them about going. If they personally would smoke, go for it. You can also phrase your original question as more neutral "I've never been to a dispensary, do you want to go?" if your worried about it, but if they do/would smoke pot they wouldn't mind if you did.
They Wouldn't, but are Accepting
If they say something along the lines of:
I personally don't like smoking, but I'm OK with the idea.
It's not something I would do, but overall I don't mind.
then feel free to ask them to come, if only for the experience. I'm not a smoker myself but I constantly hang around others who are avid fans, and I basically act as their guardian. I don't look at pot smokers as junkies and everyone I've come across with this mindset also agrees.
They Wouldn't, but are NOT Accepting
I wouldn't consider asking them or mentioning the fact that you want to go around them at all. If they start preaching about how bad it is and why it should be banned, don't bother asking. You already know the answer.