I agree with Ponhpei in that these are only points that can be made in an argument and not our personal believes
1. People say that same gender couples might psychologically affect the self esteem and the psychological development of children who naturally tend to gear towards a mother or a father by human instinct.
2. Another argument that people also use is that same-sex couples go against nature, and a child is not in a consensual age to agree to belong to an alternative family, hence, if the child is affected by the same sex relationship, allowing the adoption would have been considered government negligence.
3. One last argument that I've heard people say is that same-sex couples are simply committing a sin, and bringing a child into what anti-gays call "sinful homes" would make us equally guilty and twice as much "sinners" for allowing such a situation happen.
Again, not my believes, just arguments that people have made and can be used for your homework.