Abstract: Prophylactic devices and methods for assisting in establishing and maintaining male erectile function. Open ended elastic sheaths with portions having different elasticity than have other portions, which portions of different elasticity cause selective constriction around and/or along selected portions of the penis by differential external pressures applied to said selected portions so as to not impede arterial blood flow to the penis but impede venous blood flow from the penis and thereby establish and maintain the penis in an erectile position. Various sheath configurations for the purpose include portions with lesser radius laterally of the longitudinal center axis of the sheath, or with more rigid portions, or with thicker portions, or with a balloon-like internal chamber fillable with a liquid or gas.