I agree that 'in' seems most natural if you're choosing seats in a fairly empty cinema. But does 'sitting on the front row' actually sound wrong to you? It doesn't to me. I wonder if there's a slight difference.It would not have occurred to me to use "on".
Well, there's a difference between sitting on a chair and sitting in a chair: it depends on the type of chair. (See sit in on chair.)Is there not a difference between sitting on and sitting in chairs arranged in a row?
Actually, the question was:But this thread is about sitting on a row or sitting in a row, isn't it?
And my answer is:Would you like to sit in the front rows?
Would you like to sit on the front rows?