Sex vs. gender
Doug Fisher at Common Sense Journalism points to a cheat sheet on the differences of sex and gender.
(Note that not everyone is on the same page here. It's one of the few things that Bill Walsh and I disagree on, but I think there's value in the distinction.)
Here's how the The American Heritage Book of English Usage puts it:
Traditionally, writers have used the term Gender to refer to the grammatical categories of masculine, feminine, and neuter, as in languages such as French or Spanish whose nouns and adjectives carry such distinctions. In recent years, however, more people have been using the word to refer to sex-based categories, as in phrases such as gender gap (as in voting trends) and politics of gender. Anthropologists especially like to maintain a distinction between the terms Gender and sex, reserving sex for reference to the biological categories of male and female and using Gender to refer to social or cultural categories, such as different gender roles in a religious organization. According to this distinction, you would say The effectiveness of the treatment appears to depend on the sex (not Gender) of the patient but In society, gender (not sex) roles are clearly defined. A majority of the Usage Panel approves of this distinction, but opinions are mixed. In a sentence similar to the first one above, 51 percent choose sex, 31 percent choose gender, and 17 percent would allow both. Similarly, for the example Sex/gender differences are more likely to be clearly defined in peasant societies, 47 percent prefer Gender, 38 percent would use sex, and 15 percent would allow both words.This is probably not something you'll be expected to change in copy. However, you should be aware that the distinction exists and is not wrong.
And, by all means, you don't need to change "sex" to "gender" just to avoid giving readers a case of the giggles. Let's be a little more adult than that.