I think it depends. Personally, I’ve never felt any lust from a painting. If I look at “The Birth of Venus” for example, I feel nothing in that way. But I don’t have a particular good sense for the visual arts, though I do like the Impressionists.
But as to whether its sinful, I don’t think it has to be, but of course it could be. For example, I remember when I was a teenager, there was a certain girl I had a crush on, and I thought she was beautiful but I couldn’t have brought myself to think of her sexually even if I wanted to, because I had put her on a pedestal in my mind.
Also, a man knows when his, to quote Boswell, “amorous propensities” are being stirred, as there is a definite physiological response. So if your not having that sort of problem, I don’t think it is an issue.