Just always use the masculine as the default answer unless there is an indication that it's feminine (for example, the sound of an "e" or if you were just talking about women in the previous sentence.) French is sexist like that in which the masculine form always takes precedence. Even if there was a group of 1 million women and only 1 man, you would still use the subject "ils" to refer to the group.
The least sexist language I've found is Swahili. There is only one word for the third person singular pronoun (yeye) and one for the plural (wao). Essentially, there is one word that means either he or she, and one meaning just "they" (like the English they). You know if the sentence refers to a man or woman based on the context. Gender was not seen as important enough to differentiate between "he" and "she," let alone to characterize every object as masculine or feminine.
Edit (2 years later): I've since discovered, as those commenting below have pointed out, that having only one gender-neutral third person pronoun is not as uncommon as one might think!
Add to the list Persian as yet another language that exhibits this.
Bengali doesn't have gender anywhere at all either. "সে" (informal) and "তিনি" (formal) both mean "He/She" (sometimes means "It"); "এটা" (informal) and "এটি" (formal) both mean "It/This". Generally I dislike Bengali because of its other atrocities but this gender neutrality is what I like the most about it.
I gave it a few listens and it seems there is a very slight diffrences in pronunciation between "bons" and "bonnes", The "o" sounds very nasal in the former and very open in the latter. Check it out:
They are just "help", not correct solutions for this sentence. Consult a dictionary to get more info about different possible translations: http://www.wordreference.com/fren/bon As you can see, it means 'good' or 'correct', even in "bonne anniversaire" it means simply 'good' – but in English this phrase would be translated with "happy".
Do bon and bonne really mean "sexy" in informal french? If it's true, I noticed that in informal Portuguese it's used "gostoso(a)" for "sexy" and it means "tasty", a general word say a meal is more than "good", and in french "bon" can also refer to good tasted food so... (never say anyone is "gostoso" because it is slightly disrespectful) (anything I delete this coment)