Is remarriage adultery? No, I don't think so. Why?
Because I believe that adultery is synonymous for "cheating" and that "cheating" strictly implies sexual intercourse or activity.
--If you were to fall in love with someone else and out of love with your spouse, I believe that you are not cheating on your spouse, because how can you cheat on someone you are no longer committed to?
--If you were to fall in love with someone else and stay sexually active with your spouse you are then cheating on the other person with your spouse.
--If you were to fall in love with another person in addition to your spouse, then you are not cheating, because you still love your spouse.
On that note, I personally believe that we can and do fall in love with more than one person at a time, and that we have many soul-mates, not just one.
I also believe that marriage is a social institution that often goes hand in hand with religious observances. As such, I firmly believe in America's separation of church and state, and believe that legal marriage should be inclusive of all religious and social aspects. Monogamy, Polygamy, Same-Sex Marriage, and the like should all be included under the protection and freedom of legal marriage, even if there are certain religious qualms about these issues. I have a simple enough answer:
If you don't believe, religiously, that marriage is a holy union, don't get married in a church, don't be married by a minister or priest.
If you don't believe, religiously, that marriage is one man with many wives, don't take many wives.
If you don't believe, religiously, that marriage is two men, or two women, then don't take another man as you husband, men, or another woman as your wife, women.
However, because America is not a theocracy, I find that I am particularly vehement in my opposition to the adopted legal standing of marriage as "one man and one woman."