If people's objection to same-sex marriage is that it abandons the gender roles inherent in marriage, then they're closing the barn door after the horse has escaped. Our country now has (finally, and almost without exception) egalitarian marriage laws. The law does not require that either gender in a marriage take on a particular role withing the marriage.
Outlawing same-sex marriage does nothing to "fix" this. If their desire is for the continuation of certain gender roles in marriage, then they should change the laws to insist on those gender roles for heterosexuals - not pick on homosexuals.
When same-sex marriages are legal, people who want to have certain gender roles in their marriages will still be free to have them. Churches will still be free to insist that they will only marry mixed-sex couples. Religions will even be able to instruct their willing members in the proper gender roles within marriage. If that's the kind of thing you like, you don't need to deny gays equal rights under the law to get it.
It's easy to make up rules against gays; they're a minority. The same religions that object to gay sex also object to adultery, but there are no laws against adultery. The Catholic Church is, theoretically, as opposed to divorce and remarriage as it is to homosexuality - but it will pressure its members to vote against gay marriage, and to only support politicians who oppose it, without (in this country) applying any force to get their members to politically attack divorce. But who wants to take on the difficult task of limiting the rights of heterosexuals? Better to have a scapegoat.
