Lieu says attempts to pathologize and change people's sexual orientation should be treated akin to smoking and drinking: harmful activities that adults can choose to participate in, but children cannot.
"We let adults do all sorts of stupid and risky things, but we ban dangerous things for young people," Lieu said in a telephone interview.
He was inspired to take up the issue by a cable news documentary featuring people whose parents had attempted to change their sexual orientation. The doctor featured in the show "was evil," he said.
Interest in the religion-based therapy appears to have surged in recent years.
Exodus International, the world's largest Christian referral network dealing with homosexuality, now refers people to 260 groups across the country, up from about 100 a decade ago. The organization has 35 ministries and churches scattered around California, from the Central Valley to the U.S.-Mexico border.
Mainstream mental health organizations say people shouldn't be seeking out groups like Exodus at all.
The American Psychological Association said in 2009 that mental health professionals should not tell gay clients they can become straight through therapy.
The association cited research suggesting that efforts to produce the change could lead to depression and suicidal tendencies, and stated that no solid evidence exists that such change is possible.
The American Counseling Association and American Psychiatric Association have also disavowed the therapy. And the psychiatric association removed homosexuality from its list of mental disorders nearly 30 years ago.
Conversion therapy penetrated the national consciousness last year when former Republican presidential candidate Michele Bachmann was questioned over whether the Christian counseling business of her husband provided therapies that attempted to change gays and lesbians.
Last month, psychiatrist Bob Spitzer retracted his widely-cited 2001 study that found that "highly motivated" people could change their sexual orientation, and apologized to the gay community.
The measure would likely face legal challenges from opponents who say it is unconstitutional.
Lieu says he addressed free speech issues by excluding clergy from the legislation.
Gay rights advocates say such a ban would constitute a major milestone, and could lead to similar legislation across the country.
The debate comes as gay rights issues take the spotlight around the nation with pending legislation in North Carolina and Colorado and recent comments from Vice President Joe Biden, who said he is ``absolutely comfortable'' with same-sex couples getting the same rights as heterosexual couples.
In North Carolina on Tuesday, voters are expected to decide whether to make it the 29th state to pass a constitutional amendment defining marriage as solely between a man and a woman.
And in Colorado, a measure to extend civil union protections to gay couples faces a looming deadline in the state Legislature.