So in the evangelical Christian worldview, there are no such thing as gay people: there are straight people who succumb to the temptation to engage in gay sex, and there are straight people who either are never tempted in such a way or (bear with me) those who are but managed to withstand it. Let's ignore the fact that this is obvious bullshit and accept it on its own terms, for sake of argument.
My understanding of evangelical theology is that we, the children of God, are put on this earth to live in mortal bodies. We will be subject to many temptations from many sources, but should strive to reject the wicked ways of the world in favor of the righteous ways of the Lord. We will be judged in the afterlife for how well we achieve this goal.
It's worth considering what perspective the Lord will do the judging from. Presumably the relevant metric will be the number of temptations we didn't successfully withstand. As in golf, the lower our score, the better. But there are two ways this number could be minimized:
(1) The number of temptations could be high, but the number of times one succumbed could be low (indicating strong performance in a challenging environment)
(2) The number of temptations could be low, and the number of times one succumbed could be only slightly lower (indicating weak performance in an unchallenging environment)
It would seem to me that (1) would be more impressive to the Celestial Magistrate than (2). The earthly equivalent of (2) are the kids who graduate high school at the third-grade reading level because their school system would rather keep them in their peer group than actually educate them. (1) seems like the kind of thing that would appeal to a real conservative.
Given this, wouldn't it be in the evangelicals' long-term spiritual best interest to ensure that the ways of the world remain wicked, so that their own spiritual mettle can be proven as in case (1)? By fighting against the homosexuals (and the pornographers, and the prostitutes, and the liberal Hollywood elites -- this line of reasoning works for a whole bunch of evangelical targets), aren't the evangelicals essentially childproofing the house of God, and removing all moral challenge? Doesn't this subvert God's very purpose in having put us here, to prove that we are worthy of being saved?