LAST week it was nude pictures for donations - this week People for the Ethical Treatment of Animals wants to neuter a man.

Are they fur real?: PETA's online vasectomy offer.

PETA is offering the chance to win a vasectomy, saying "Human overpopulation is crowding out animal life".

"Now, one lucky man can be reproduction-free, free of charge," PETA's website said.

The "winner" has to have had his dog or cat neutered recently.

The vasectomy offer comes as women's groups this week slammed PETA for objectifying women, while political branding specialists warned the animal rights group could alienate its left-wing base.

Author and women's advocate Melinda Tankard Reist said PETA's latest stunt - offering naked pictures of model Vida Guerra for $5 donations - would see a "huge backlash".

"I know many people who pulled (support for PETA) over that," Ms Tankard Reist said. "They're demeaning, degrading and exploiting women in the cause of degraded and exploited animals."

Victorian Women's Trust executive director Mary Crooks labelled the donation-for-nude-picture stunt "pathetic". "The bottom line is it defines (a woman's) value by the fact ... she has boobs," she said.

Women's Network Australia founder Lynette Palmen said PETA was "appealing to the animal instincts of humans to raise funds for animals".

Australian National University marketing specialist Andrew Hughes said PETA had "absolutely" damaged its brand and alienated part of its base.

Mr Hughes said the model's consenting to the use of the image was beside the point because the stunt was "asking cash for a nude image".

"A lot of their market are women who are absolutely left-wing - that's where their space in the marketplace is," he said.

PETA spokeswoman Ashley Fruno said there was "no exploitation when an individual driven by her beliefs chooses to use her freedom of expression, via her own body, to bring attention to animal suffering".

Ms Fruno has "proudly" bared for PETA campaigns.

"Since the press and public pay more attention to racy and colourful ads and campaigns ... I choose to use my body as a political tool to draw attention to a good cause," she said.