What does the expression "孤男寡女" (gū nán guǎ nǚ) mean, exactly? Does it have inherently negative connotations?
As an aside, why does Google translate it as "Needing"?
Frankie's answer is good, but I want to make a clarification on 孤 and 寡. Both of them mean "only one" here, and I don't think "single" is good in this situation since it could mean "unmarried".
you and a female friend of yours are taking in a club, in a room with the door closed,(of course, I don't recommend this :-) ), unfortunately, your wife finds you here, so she would high suspect that you have affairs with your friend.You try to explain, but she retorts,
你们孤男寡女，共处(chŭ, to stay in )一室，你还有什么好说的？
one man, one woman, and one room, how could you explain?(I won't believe!)
The phrase, in direct translation, means single guy & single lady. More often than not, it is being used to implying that when a single guy & single lady spending time together alone, bad things/gossips might just happened.
It describes a man and a woman being together (usually in some manner of seclusion). Depending on the marital status of the man and the woman, it can be seen either as a good/normal (a couple dating) or bad thing (a brewing affair). There is no inherent negative connotation unless it is used in certain context.
Google is just confused. It has problems with 寡女 (somehow it was "trained" into translating that into pinyin equivalents
guanv1) but not with 孤男,寡,女.
A literal translation is "lonely man,lonely woman." To translate it as "needy" is a bit of a stretch, but that's basically what a lonely man and woman (together) are.