I don’t remember where I saw it, but I read somewhere that BB-8 is supposed to be female (s/he has a different “voice” than R2, at least).
Is there any evidence supporting either gender for BB-8?
There is no gender clearly specified in the film to my recollection (this part is subject to review as I re-watch, or as official script is available).
BB-8 is referred to as "he" in the Alan Dean Foster novelization, so the answer is "not female".
Except that sometimes it's an ungendered "it" too.
Where a human would see only empty night sky, advanced calibrated synthetic optics saw a moving point of light. When the light resolved itself into four separate points, the droid commenced an agitated beeping. The phenomenon he was seeing might signify nothing, except…
Beeping and whistling in something approaching cybernetic panic, the droid spun and sped back toward the village. That is, its head spun... While it could have transmitted the conclusion it had reached, it did not do so for fear of any such message being intercepted
So BB-8 was forced to dodge and avoid, which he did with skill and patience,...
Deep within the village, there was far less likelihood of encountering anything domesticated that was worth eating: a biological process he understood from an objective point of view but for which he could never rouse much empathy. His goal was close, and there was not a nanosecond to lose.
...Far away now, a solitary spherical droid looked back even as it continued to flee. The fireball that rose into the sky suggested the detonation of something far more volatile than primitive buildings and scrapped mechanicals. If he could have rolled faster, the frightened droid would have done so.
... There were things in the vacant, wild regions of underdeveloped planets that would gladly take apart a solitary droid just to see what made it tick. Or roll, he knew. His internal gyros threatened to send him tumbling wildly at the very thought of such an encounter.
Droids such as him were not meant for unpopulated places, and he desperately desired to find others like himself. Or, failing that, even people.
No gendered info in Pablo Hidalgo's Visual Dictionary page for BB-8.
As the Facebook status goes, "it's complicated".
On one hand, JJ Abrams was reputed to say:
JJ [Abrams] was determined to make BB-8 cute and strong – and female. They want to appeal to girls as much boys, who have traditionally been the fan base. She’s going to be one of the breakout hits of the film.”
Same source says:
Lucasfilm president Kathleen Kennedy later referred to BB-8 as “she”, and reports from the films set, as well as some of the Star Wars publicity materials, suggested that the new droid was a girl (confusingly, puppeteers Dave Chapman and Brian Herring, who are responsible for operating the robot, have previously referred to BB-8 as “him”).
However, same JJAbrams called BB-8 a "he" in Entertainment Weekly.
Abrams chose the droid’s name because it looked round and bouncy. “I named him BB-8 because it was almost onomatopoeia,” the director says. “It was sort of how he looked to me, with the 8, obviously, and then the 2 B’s.”
All the acting "talent" behind BB-8 was male:
However, from what I could tell from Ben Schwartz's interviews, most of the "talent" came out of iPad app, which again supports "no gender" status as iPads aren't gendered.
(@rand's answer has more complete details of the "complicated" status :)
This quote (published on 13 November 2015) from Neal Scanlan, the head of the Star Wars: The Force Awakens creature shop, sums up the confusion rather nicely:
I’m still not sure, dare I say, whether BB-8 is male or female. BB-8 was female in our eyes. And then she became male. And that’s all part of the evolution, not only visually, but in the way they move, how they hold themselves.
An anonymous "source close to the film makers" said:
There’s never been a strong female robot in any Star Wars film. [JJ Abrams] was determined to make BB-8 cute and strong – and female. They want to appeal to girls as much boys, who have traditionally been the fan base. She’s going to be one of the breakout hits of the film.
According to the same article, published 2 November 2015, Lucasfilm president Kathleen Kennedy also referred to BB-8 as "she".
JJ Abrams referred to BB-8 as male in an interview published in December 2015:
Yes, BB-8 is a great character, amazingly puppeteered, but what will happen when he’s suddenly in a scene with C-3P0 or R2-D2? Will it feel bizarre? Will it feel wrong? Somehow it didn’t.
Brian Herring, one of the puppeteers running BB-8, also referred to him as male (published 26 August 2015):
BB-8 can cock his head over and look away, he can double take, he can look scared, he can look angry. We managed to find a whole vocabulary of movement for him, if you will. We worked out a whole bunch of stuff. What would he do if you turned him off? What happens to his head if you power him down? Does he go down stairs? Does he go up stairs?
It's possibly worth noting that the puppeteers who run BB-8, Brian Herring and Dave Chapman, are both male. So in that sense the droid is 'played' by men.
Interestingly, this question was recently posed to the cast and crew. Results were mixed, with most saying they never thought about it or saying as a droid BB-8 was genderless. JJ Abrahams stated
I always refer to him as a 'he'. But I don't know, I haven't looked under the hood.
You can watch this segment here: