The sexual urge is so strong that sometimes a man indulges in sexual relations with a cow, or a woman indulges in sexual relations with a dog. Such men and women are put into the hell known as Vajrakantaka-salmali. The Krsna consciousness movement forbids illicit sex. From the description of these verses, we can understand what an extremely sinful act illicit sex is. Sometimes people disbelieve these descriptions of hell, but whether one believes or not, everything must be carried out by the laws of nature, which no one can avoid.

Srila Prabhupada, SB 5.26.21, ppt