I've seen young people turn into gormless idiots after prolonged smoking. I've seen young people continue to be intelligent, ambitious, motivated and successful while smoking. It's unreasonable to make such sweeping generalisations, when not everyone is affected in the same way.
I much prefer the "homegrown shite" to anything imported, because I know its production hasn't funded the international drug trade, and I know nothing's been added to it. Not convinced homegrown stuff fucks one up more than imported stuff; could you clarify what you mean by that?
Most of the people I know who use it find it helps them to relax, or it helps them to sleep. Person A, crippling arthritis, smokes a little at bedtime because otherwise the pain's so bad he can't sleep. Person B, suffers night terrors, wakes her partner up screaming sometimes. Smoke before bed and she sleeps fine.
You're not being unreasonable to want a partner who doesn't smoke weed, though. It's entirely your decision. Just bear in mind that you can only change whether he's your partner, not whether he chooses to smoke. Telling him he can't smoke it any more is indeed bossy and controlling, and he'd be well within his rights to tell you to sod off. Having a conversation in which you make it clear that you don't like him smoking and see if he's willing to stop would be a much better idea.