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On most major issues Americans tend to split in black and white terms.

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I don't think so.

Abortion: You're either for it, or against it. Or against it unless the mother's life is in danger. Or perhaps if she was raped. Or the morning after-pill is OK - or not, because conception has already taken place. Or even birth-control should be illegal because it doesn't prevent conception. Or abortion before the third trimester is OK. Or lots of other positions, I'm sure. Major issue. Lots of positions.

Gay Marriage: For it, or against it. Or against civil unions too -- or for them. Or against the government being involved in marriage at all. Or it should be left up to the states, and other states should honor marriages in other states -- or not honor them. Or it should be made illegal, or legal, at the federal level. Change the constitution, or don't change it. Major issue -- lots of positions.

I will grant you that Republicans tend to be more cohesive in their beliefs. So, "Republican" vs. "Not Republican" would probably work better than "Democrat" -- because Democrats tend to have a lot more diverse opinions than Republicans. (Yes, this is a generalization, not an absolute.)