At least he had his curtains open, OP. 
It's another parallel universe moment for me though. We KNOW it's his house and if he wants to stand 'avin a fag in his pants, that's his business etc., and on MN, it's obviously a total no-issue.
In the real world, or at least the one I live, in, it's not particularly normal, no. Not illegal, but not exactly usual. But if it's a one-off, whatever, it's a nice sunny day.
If it's not a one-off, kids around, maybe on school route or something, and a bloke stands there in his pants as they all walk past?
That wouldn't seem okay to me, even though IT'S HIS HOUSE, only like swimming trunks/what exactly do you THINK is going to happen, OP?.. etc. etc...