Same sex marriage bans cause mental stress for gays and lesbians, according to a US study.

The research, “I do, but I can’t,” at the National Sexuality Resource Centre, in San Francisco, found that denying same-sex couples the same marriage rights as heterosexuals created “the mental distress of second-class citizenship.” They claim bans increases depression and isolation.

One of the authors, Gil Herdt told the Agence France-Presse, “Marriage denial creates what experts call minority stress, the psychological effects of constant discrimination that bars individuals from the legitimate means of achieving goals that are valued by the society in which they live.”

“Lesbians and gay men work just as hard as heterosexuals do in creating and maintaining committed relationships, but they do not get the same tangible benefits.”

The report comes just when Christian groups, lawmakers and residents in several parts of America are debating gay marriage issues such as in Boston, Wisconsin, and Minnesota.