I suspect it isn't so much the erectile function as the shrinkage. When the body is cold, the blood tends to remain closer into the centre of the body - hence your fingers, toes, nose and 'other extremities' get cold (hence the 'Oh look, a willy but smaller!' type comments).

Boobies get cold, the blood retreats to the centre of the body and the nipple tissue contracts, due to lesser blood flow. If you get the chance, have a look at chilly norks... you'll see that the surrounding aureole is smaller and wrinkled.

It's very, very difficult to breastfeed (or express milk) with cold tits.

LTS