There are a couple of points worth making in this argument, which so far no-one has pointed out
1) the "civilians" in the bombed city were to a large extent the workforce of the target factories, and therefore legitimate targets themselves. BY bombing the cities, you destroy or harass the workforce, and destroy much of the infrastructure which makes the work possible: the roads, buses, trams, power and water supplies
2) much of this derbate seems to assume that german industry was built in discrete isolated industrial sites. That wasn't the case. Just as in the UK, the industrial development of the 1800's had brought manufacturing into the towns, with houses filling the gaps in between. Go into a British town like Widnes, or Manchester, and see how the metal-bashing and chemical plants were cheek-by-jowl with the housing. OK, there were some dedicated plants e.g. the Ruhr steel works, but in the main, most German industrial production took place withing the cities, alongside the workforce. You could not destroy one without destroying the other
To attempt to suggest that bombing could be kept to "industrial targets only is pure nonsense. The towns and cities WERE the manufacturing factories, there was no way not to hit them. Any pretence otherwise is just muddled thinking by modern bleeding hearts liberals who have never taken the time to actually look at the history of manufacturing industry and the industrial revolution