Humans having sex with animals, otherwise called bestiality or zoophilia, is fairly uncommon, or uncommon to tell the researchers, in this century. It is thought that when more people lived on farms and in rural areas, the practice of having sexual relations with animals was much more common. Most people who have sex with animals are men, but there have been some documented cases of women caressing and stimulating animals in an attempt to insert the penis of a dog or horse into their vagina.
Human sexual relationships with animals have been a topic of interest for people for hundreds of years. Tales of human-animal contact can be found throughout ancient folklore and mythology. A good example is the story of Zeus, who in the form of a swan, had sexual intercourse with Leda, the queen of Sparta. William Butler Yeats used this story as background for a famous poem. Greek and Roman mythology have portrayed females having sexual relationships with bears, apes, bulls, goats, horses, wolves, snakes, and crocodiles. Humans have enjoyed rich sexual fantasies throughout the ages, sometimes including the fantastic and the unreal.
Fact: Of the almost 6000 people females interviewed for the Kinsey comprehensive 1953 study on sexuality, only three reported that they had had sexual intercourse with an animal, two as preadolescents with dogs, and only one as an adult. Another study done in 1974 found no women who had experienced intercourse with an animal (Hunt). The percentages of adult human females who have had any type of sexual contact with animals are low -- 3.6 percent in the Kinsey study, and 1.9 percent in the Hunt sample. The animals involved were most commonly dogs and cats, and the sexual activities most often reported were general body contacts with the animals, and cunnilingus performed by animals.
Male animal contacts are more common, although the total percentages still remain quite low. Eight percent of Kinsey's sample brought themselves to the point of orgasm with an animal, and 4.9 percent of Hunt's sample did. Male sexual contacts with animals were more common among rural farm dwellers than urban boys. Coitus was the most common sexual activity, usually with animals, such as calves, sheep, and burros. For both males and females, their encounters with animals were most likely to have occurred before puberty, and to have been sporadic encounters with little consequence on the people's sexual development.
Historically, taboos against human-animal contact have been quite severe. As recently as 1953, 49 states regarded bestiality as a felony or its equivalent. In eight states, the maximum penalty was life imprisonment. Many such laws remain the same to this day.
According to the stats Alice gives here, it is not a very common thing to use animals for sexual pleasure.
Regarding your guilt over this, I think the only thing you can do is move forward. You changed your mind about what you did back then and now think it's "nasty", so try to accept that that's the past and recognize that you can't change it anymore, but you can move on.
If your guilt really stresses you out, it might be a good idea to discuss it with a professional, such as a psychologist or counsellor, to work out a way to move on.