Either way JWs face real problems. The trends are clear that gay marriage is increasingly accepted by all sections of society. It has followed a similar trajectory to views on mixed race marriages, which very few now oppose and opposition is socially unacceptable.
If current trends continue - and the overwhelming support for gay marriage among you people suggests that it will - then JWs will soon be very isolated. A very similar position to the one the Mormon church found itself in regarding race equality in the 1970s.
Evangelicals are already rapidly accepting gay marriage. In some ways they'll have an easier time of it than JWs. Evangelicals are very interested in politics, and as it becomes politically untenable to oppose gay marriage that will be a big incentive for them to change. Another thing is that power is more evenly distributed among the Evangelical community, so there is scope for pastors and individuals to draw their own conclusions and change at their own pace. JWs on the other hand need to wait for the GB to change any policy, and there is a strong incentive for GB members to stick by their previous statements on the issue as long as they are alive.
There are ways to interpret the NT that allow for gay relationships. Many Protestants already do this. JWs already interpret away many many scriptures. For example the scripture that says a sinner must present himself to the congregation. JWs, with no shame whatsoever, say this verse means the sinner must go to the elders, not the congregation. That is as flat a contradiction of the plain sense of scripture as you could hope for. They are more than capable of explaining away and adapting verses on homosexuality if they so wish.
Will they adapt or won't they? Who knows, really? But there are no easy choices for them either way.
I would argue that if attitudinal trends continue, then not adapting will become simply too difficult to maintain, as it did for the Mormon church refusing to admit black people into the priesthood. However painful the climb down may be.