What Causes Dark Purple Spots On Penis Glans?

I noticed dark purple spots in the glans of my penis about a month ago. There are two on the side of the penis, near the frenulum. Then there are several such spots around the tip of the penis. They do not hurt or bleed or itch, but my wife is a little freaked out about them, and I don't love it either. They get darker and stick out more when erect.