Opponents of same-sex marriage will announce tomorrow an initiative petition to put a constitutional amendment on the statewide ballot in 2008 that would ban such marriages, lawmakers and activists involved said yesterday. Activists and strategists familiar with the effort to ban same-sex marriage, who spoke on the condition of anonymity, said they believe their proposed amendment would have a better chance of winning approval because it contains simpler language that defines marriage as between one man and one woman, without mentioning civil unions.