If a man is to be married in the near future, should he give up doing personal stuff by himself, or is that OK even after marriage?
Not only is it okay occasionally, BUT it is RECOMMENDED. Marriage should not be a prison sentence.
I think it is HEALTHY for a couple to have 'personal stuff' as well as 'together stuff'. The hardest part of our marriage was when we both worked from home. We shared a study, so we were basically in the same room 24/7. And it drove us absolutely bonkers. We work far better when we spend some time apart - sometimes even overnight - and then reunite and actually have things to talk about. And even when we're together we each have our own hobbies. He's into electronics and computer programming, I have taken up sewing and loom knitting. We take an interest in each other's hobbies without actually expecting the other to share them.
He should be able to do what he wants as long as he's not cheating
Most married couples do their own thing. The two of them should discuss what their expectations are.