Defense Secretary Robert Gates is urging gays in the military to answer a Pentagon survey on the policy that bans them from serving openly, but advocacy groups worry the poll may be biased against gays or that those who participate could be exposed and expelled.

"I strongly encourage gays and lesbians who are in the military to fill out these forms," Gates said of the poll e-mailed to some 400,000 soldiers, sailors, airmen and Marines. "We organized this in a way to protect their privacy and the confidentiality of their responses ... and it's important that we hear from them as well as everybody else."

Gates told a Pentagon press conference that the survey — distributed Wednesday to 200,000 active duty military and 200,000 national guard and reservists — is crucial to an effort to broadly measure the views of men and women in uniform on the policy.

The Servicemembers Legal Defense Network said earlier Thursday that the Defense Department has not agreed to grant immunity to anyone inadvertently outed during the survey, intended to help a special working group decide how repeal of the policy might be implemented and how that could affect the military.

"At this time SLDN cannot recommend that lesbian, gay, or bisexual service members participate in any survey being administered by the Department of Defense, the Pentagon Working Group or any third-party contractors," Aubrey Sarvis, the defense group's director, said in a statement.

Another group, Servicemembers United, said it was concerned about "unintentional bias" in the wording of survey questions but was satisfied it would not violate the confidentiality of participating gays. The Pentagon is not publicly releasing the survey, but some draft questions the group had learned about were homophobic, director Alexander Nicholson said in an interview. For instance, one asked troops if they would be comfortable sharing bathrooms with gays and lesbians.

"Servicemembers United encourages all gay and lesbian active duty troops who received the survey to take this important opportunity to provide their views," Nicholson said in a statement.

A third group, the Palm Center, did not object to the survey, saying it was part of an agreed-to process of dismantling the "don't ask, don't tell" policy under which gays can serve as long as they don't reveal their sexual orientation.

Aaron Belkin, director of the center, said his group did not expect the survey to support repeal. "That said, we welcome the results and value the feedback of all the troops," Belkin said. "We will pay close attention to this process."

The Pentagon says the survey is being conducted by an outside contractor who will strip out all identifying data. "They cannot be outed," said Cynthia Smith, a Pentagon spokeswoman.

Smith noted that the survey doesn't ask whether a respondent is gay. It asks, among other things, about service members' experience in serving with people they believed were gay and their attitudes about how repealing the law might affect recruiting, privacy and unit cohesion.

Officials stressed that the survey is not a referendum.