The Sistine Chapel was strict on dress codes. It was only 18C the day we went so I wasn't inordinately uncomfortable in my lightweight long pants (cheapies bought specially for the trip). Elsewhere in Italy you are sort of expected to cover up a bit in churches, but I find that the rule is very patchily applied. If they think you might spend money, you're in as long as you're not exactly bollock-naked. Occasionally an old lady at the door will provide a modesty wrap for women's bare shoulders. Ordinary shorts, as opposed to Daisy Dukes, are almost always OK everywhere. I suppose it's a risk turning up to some holy place unprepared, especially if it's one you dearly want to see. In dozens of visits to churches all over Italy, I've worn shorts, the Sistine Chapel being the only exception. But then I'm just a very unsexy-looking bloke. A good idea is to have a pair of pants that have zip-off legs that you can keep in your bag, just in case. I did that in Venice but never needed to zip them back on!