Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 382
Question: What is the ruling regarding the gathering together of men and women in the same place? And likewise what is the ruling regarding their gathering together to eat?
Response: As for the gathering together of the family in the same place – with men on one side and women on the other, then there is no harm in that so long as there is no fitnah we are aware of [which may emanate] from some of the men or women, in which case the gathering should not occur. Other than that, the basic principle is one of permissibility.
As for [men and women] gathering together to eat, then this is not correct. [In such a situation] it may be that a woman sits next to a man who is not her mahram, nor her husband. Also, how will she eat if she is maintaining her hijaab (niqaab) over her face – which is necessary to be uncovered – since it is obvious the food is [to be] put in the mouth, [something] which necessitates the uncovering of the face?!; Because of this [reason], we are of the opinion that such a gathering should be prevented.
As for the presence [of men and women] in the same place and gathering – or in an auditorium, then there is no harm in that, so long as the men are separated from the women.
 It is clear from the rest of the Shaykh’s response that he means the immediate family, in front of whom the women are not required to cover their faces – namely their husbands and mahrams, wAllaahu A’lam.