[What happens to people of faith who support traditional marriage after the legal definition of marriage is redefined as anything other than a union between one man and one woman?]
Not a whole lot.
We have first amendment protections for speech and religion, and a large bloc of Christian voters.
Christian business owners that cater specifically to weddings may have to be wise as serpents if they wish to decline gay clients, but pastors and churches will be fine.
Christian business owners that do not fall into a specific religious vocation which provide benefits to married employee’s spouses will have to do the same. Whether or not you would find to be an intolerable religious compromise or not will depend on your conscience.