Conservative Christians and Catholics are not going to go along with abolishing the institution of marriage out of spite for same-sex marriage. The status between a married couple is much bigger than any contract that the couple can come up with themselves. For example, a spouse is legally the next of kin, a spouse gets pensions and social security benefits after the earning spouse dies, spouses can file joint taxes and joint bankruptcy, a spouse is responsible for all the debts incurred by the other spouse, including medical care, spouses are legally required to support each other, etc.
Also, I am a married man and abolishing marriage is a personal attack on my status and the status of my wife, and this goes deeper than man-made law. Marriage is a sacrament of the Catholic Church, and in Genesis it says that a man and wife become one flesh in the eyes of God, and the bond is not broken until death.
First of all, Christians and Catholics are out of choices if they wish to retain sanctity of marriage. If it remains a state sanctioned institution, it has to include homosexual marriage. .
Religious institutions can perform marriage- but religious marriage doesn't have to include mixed race couples or Jewish couples or gay couples.
Christians and Catholics can deny marriage within their religion to Jews, to divorced people, to gays, to Muslims, to Mexicans- they can deny marriage within the church to any group that they want to.
Just more fear mongering and lies by homophobes.