Germany is to introduce a ban on bestiality with a revision of animal welfare law that will reverse a decision in 1969 to legalise zoophilia.

Animal rights groups have called for the practice to be recognised as animal defilement and rape, using dramatic photographs of animals being cruelly treated by humans for sexual purposes to put pressure on the German government.

Zoophiles, or those who practise bestiality, argue that they treat animals as equals and never force them to do anything against their will.

"We are going to take legal action to fight this," said Michael Kiok, chairman of the pressure group Zoophile Engagement for Tolerance and Information (Zeta), which has about 100 members. "Central to the beliefs of zoophiles is that we don't do anything that the animal doesn't want. We do not treat them cruelly. An animal is quite capable of showing precisely what it wants and does not want. When I look at my dog I know immediately what it wants. Animals are much easier to understand than women," he said.

The centre-right government of Angela Merkel, which has shaped the law change, said key to the new legislation was recognising that sex with humans was "inappropriate" for animals. According to Hans-Michael Goldmann, chairman of the parliamentary agricultural committee which will discuss the law on Wednesday before a final vote in the Bundestag on 14 December, animals should not be used "for personal sexual activities or made available to third parties for sexual activities … thereby forcing them to behave in ways that are inappropriate to their species".

Animal rights' campaigners welcomed the ban, but said it was being used to divert attention from more widespread and pressing animal rights issues. "We welcome the new amendment (to the law) to ban sodomy," said Thomas Schröder, president of the German Animal Welfare Association (DTSB). "But this achievement in the animal welfare struggle cannot be used as an excuse to conceal other urgent animal rights questions".

Agriculture minister Ilse Aigner was said to have introduced the amendment to appease animal rights campaigners angry that more is not being done to tackle perceived animal rights abuses in the agricultural industry. Campaigners wants bans on cutting off piglets' tails, and the castration of piglets and branding of horses without anaesthesia. In the end Aigner has affected a change only on the issue of castration, but not until 2018, when the practice will only be allowed to be carried out under local anaesthetic.

Kiok, 52, a university librarian from Münster, southern Germany, who lives with his 8½-year-old Alsatian called Cessie in what he calls a relationship, said zoophiles had been "put on the sacrificial altar of a lazy compromise". Estimating there are about100,000 active zoophiles in Germany, he said the group remained deeply misunderstood. "We don't have anything to do with people who abuse animals," he said. "We only want what's best for the animal. "It cannot be that the mutually gratifying sexual contact between human and animal is forbidden because it's unfavourable to the species, but the abuse of animals in the agricultural industry continues."

He said he had received death threats from campaigners who were threatening to remove his dog. The protesters posted on Facebook that they had the addresses of 800 zoophiles and would campaign to have their pets taken from them.

Koik said he had been through years of therapy and despair due to his zoophilia, only discovering on the internet the "huge number of zoophiles, especially in the US," he said. "It was a relief to feel I wasn't a monster as I'd heard for much of my life," he said.