I think, from years of personal experience and observations, that yes, the use of marijuana can cause some individuals to lose motivation. In the cases I have personally observed, there have always been other underling issues, namely, depression of some sort.
My ex went something like this. We met when he was going through a bad divorce. He smoked occasionally, but still got up every day and went to work, did what he was supposed to do. He was a midnight toker - you know, get all the things done you have to get done then have a toke or two to unwind from the day and relax. Divorce was finalized, and alimony and child support together were 60% of his paycheck before taxes. So, that left him with very, very little money on each paycheck. We were living together at that point, and I had to pick up a 3rd job just to make ends meet. That made him more depressed. So, he started smoking more and more. He lost his job. He told me after he lost his job that he would smoke a bowl on the way to work - so he was staying high all day by that point. He didn't look for another one. I tried to be encouraging, I tried to help him find a job making better money (which was very probable, considering his background and resume) where he would actually have some take home pay after the alimony and child support came out. I gave up after a few months and left him; I moved several hours away. A month later he wanted to join me, but I told him only if he stopped smoking. He said he would. He came up, got a great job where he was actually bringing home some money, and things were fine. Until life got hectic again (ie - our son was born). Then, he started smoking again. Went to work late, or didn't go at all. Eventually lost that job. I left.
Now, over 3 years later he once again has a great job, lives a few hours away from me, and is actually paying me child support for the first time in years. But, he confided in me a few days ago that he has started smoking again. I know that now its just a matter of time before he looses this job, too.
I have also know people who can smoke pot first thing in the morning, maintain a good buzz throughout all of their waking hours, and still do everything they are supposed to do and then some. My father is like this. Greatest guy in the world, brilliant, highly regarded in his field, chronically responsible, bulldog ambitious. He can partake as often or as rarely as he wants, and nothing about him changes. He's a little more laid back and approachable when smoking, but he still is the same authoritative, ambitious man he has always been. And he has no idea that I know that he smokes.
So, I guess my opinion is that yes, smoking mj can cause some people to lose all ambition - but, I think there are usually bigger underlying reasons there. I think it all depends on the person. Most people who smoke know how it affects them, and the grounded and responsible ones adjust their usage accordingly.
I am a smoker, but I'm a midnight toker. After I have done all of my housework, attended to my son's needs, fed him, read to him, put him to bed, etc., then I might occasionally smoke a bowl to unwind. I know that smoking causes me to be paranoid around anyone but my closest people, so I would never smoke and then try and enter a social situation of any kind, esp work.
I sincerely hope that you are able to have a productive conversation with your partner about this so that you two can reach a compromise of sorts, and plan for your future together.