Wherever same-sex marriage has been allowed, no noticeable or measurable effects have been observed on heterosexual marriages. It does not protect marriage in any sense. It merely says that same-sex couples may not have their legal marriages recognized by all states if some states decide to grant such rights and responsibilities without regard to the sexes of the partners. But now even the most conservative religious groups recognize that sex and marriage can be for non-procreative purposes. If marriages can legitimately be directed toward sexual fulfillment, then lesbians and gays also qualify.
Every society ought to have laws protecting children.
But heterosexual marriage as the only context for raising children has now largely become a pattern of the past. Less than half of American children live with both of their biological parents. Adoption is permitted by single people and increasingly by same-sex couples. Children don't need fathers as much as they need non-abusive, involved, caring parents. And, of course, gay and lesbian people can be good parents.