why is 'he is my future man' marked wrong?
The Swedish sentence only ever means "husband" in this case, but English doesn't really use "man" in that way so it's not an idiomatic translation.
Could this also mean 'becoming'?
No, not really.
it does not take "he is my soon to be husband", is it a weird sentence or just an incorrect translation?
Well, it doesn't say anything about soon. :)
fair enough. Thanks