[jrmg]

'They' is perfectly acceptable, and has been for hundreds of years.

From http://www.merriam-webster.com/dictionary/they :

The use of they, their, them, and themselves as pronouns of indefinite gender and indefinite number is well established in speech and writing, even in literary and formal contexts. This gives you the option of using the plural pronouns where you think they sound best, and of using the singular pronouns (as he, she, he or she, and their inflected forms) where you think they sound best.