''For homosexuals to win, Bible believers have to lose''

Take the evolution/God example you use. One option is to let each teacher to teach whatever he desires on the subject. The result would be that both sides would be taught in proportions roughly equal to the people in society who believe in each.

Another option would be to require that both be taught. Give the arguments for and against each and allow the individual to choose.

In either case one side does not win and the other lose.

Now consider human sexual behavior. It is wrong, from a Christian perspective, to have sex outside of marriage. Does this mean all such activity should be punishable by the government. The story of the woman caught in adultery suggests otherwise. Such behavior is wrong, but it is for God, rather than civil government, to deal with the sinner.

Homosexual behavior, like adultery and other types of fornication (heterosexual or homosexual), is wrong, but it doesn't necessarily follow that it should be banned, or that a refusal to ban it necessarily is a loss for Christianity. Lying is wrong, but the government doesn't punish all lies. Does this mean Christianity loses?

Homosexual "marriage" is a perversion of the institution as created by God, but any form of sex outside of a man/woman marriage is as well. So long as Christian institutions aren't forced to support, perform or recognize homosexual marriages, then Christiany does not necessarily lose. Perhaps the problems that inevitably result from the sinful behavior will lead to the opening of hearts to the gospel message allowing more souls to be saved.

I am not attempting to argue for homosexual marriage or increases in public support of their behavior. Sin leads to consequences for the individual as well as society. Because of this it is wise to shape government in a way that sin is discouraged.