My marriage, being freely entered into and continued because it brings us both contentment, is not threatened by the opportunity for others to form their own bonds, however they may want to negotiate them. I am concerned for people who enter into marriage without reflection and discussion, but if they're legally capable of consent, that's up to them.
I believe that no one should be forced into marriage or forced to stay in a loveless marriage; neither should anyone legally capable of consent be prohibited from entering into a marriage of their choice. (I do have some concerns about consanguinity, but since pregnancy is no longer an inevitable consequence of sexual activity, I'm willing to reserve judgment there.)
I believe that religious denominations have the right to decide who may or may not be married under the rules of their church, but that they do not have the right to decide who may or may not be married under the rules of other churches and the laws of the state and of the nation.
I believe the "full faith and credit" clause of the US Constitution should override any state attempts to refuse to recognize marriages performed in other states.