fence sitter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who takes a position of neutrality or indecision, as in a controversial matter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who takes neither side of an argument or controversy, but maintains a neutral position.
• noun potentially offensive One who is bisexual, or who is uncertain about his or her sexual orientation.