Let's say a person, an adult, really wants to marry a minor. Should they be able to?
No. And there are adequate laws defining what a minor is and is not allowed to do as well as how unrelated adults may or may not interact with a minor. Not an issue. It is addressed.
Another person, an adult, wants to marry their adult sibling and do so for financial/inheritance reasons. Should they be able to?
No. And there are myriad (though differing) laws addressing incest and marriage of relatives at the state level. Again, not an issue because it has been addressed.
Another adult male wants to marry 2 or more adult females. Should they be able to?
In this country no. And there are laws against it. In other countries this would not be a problem because it is allowed.
In all of the above situations I'm sure that both parties may "really want to" get married, yet there are restrictions on those marriages.
So, if the current restriction on gay marriage is lifted I would expect a fight for other restrictions to be lifted, too. Maybe not tomorrow, but someday.
. You are probably right. There would be challenges. Those challenges are (IMHO) much easier to deal with than the same sex issue. As I've stated before, there are laws that don't allow
anybody to engage in the behaviors you point out. The whole point of the same sex issue is that by simply disallowing adult same sex couples to enjoy the legal benefits of legal marriage, they are discriminated against with respect to tax laws, inheritance laws, and other employment benefit / health care laws. That condition does not exist in any of the other situations you've described.
Yet I digress. The Bible clearly says that right will be called wrong and wrong will be called right, so I'm not surprised at the rapid pace at which gay marriage is gaining in popularity.
Again, no argument on that point. Thing is, the case is not being argued at the Vatican or the National Baptist Convention. It's being argued at the Supreme Court of the United States of America and needs to be argued on points of law, not relevance to Biblical position or impact on other laws (against sexual involvement with minors or incestual behavior/marriage) that are not being challenged. Those challenges need to be handled if / when they occur. You can't base a legal decision on Issue A on the potential that it might cause someone to challenge Issue B.