How will same-sex marriage impact different-sex marriages?
It won’t. Allowing marriage licenses to be obtained by same-sex couples will have no effect on different-sex marriages. People always enter marriage as two individuals; they stay in their marriages (or not) for personal reasons. The benefits which different-sex couples own thanks to legal marriage cannot be diminished by affording the benefits equally to other, same-sex couples. A legal, freely entered marriage by a same-sex adult couple is a validation of the concept of marriage, not a detraction of it, individually or in general.