I voted for the second option before I saw that the OP said “and get off me.” If I could change it, my answer would be to withdraw immediately.
I’ve had this happen a few times, and while the correct answer is the first one, the reality is the first few times, the 2nd one occurs.
“Ok, stop.”
“Wait, what?” *stop moving, stay inside
“Really, I’m done. Stop.”
“Um… am I hurting you? I haven’t finished yet.”
“Yeah, sorry about that, but if you keep going it’s going to start hurting.”
So withdrawl, and she rolls over and goes to sleep.
I trained myself to finish at the same time or rather quickly after her cumming, otherwise getting to finish at all would have been a challenge.
Pretend you didn’t hear her 
Not enough information to decide. Was it an emphatic STOP! or a barely discernable stop? Has she done this before, and what was the situation? Have I done her before? I’m not going to disregard her, but the context would make some difference in how fast I react.
Shit, that sucks. Whoever comes first should at least help the other person get off, even if they have to stop the intercourse.
Just for you.
She’s one of the women who has charged Assange in Sweden.
Until this whole Assange thing I never realized how many times I’ve been raped in my life.
Does it count if you don’t really FEEL raped? There are several instances in my memory when I asked someone to stop, usually because I’d been asking meekly the entire time but then a few others when I’d just gotten sore or needed to go to the bathroom. At least once I was so messed up I wasn’t completely aware until there was penetration.
Yeah I had a few bad years in my youth.
Okay, who is the one would voted couldn’t stop?
Is it ok if I switch holes?
As a male, here are a few I’ve encountered:
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Your college roommate decides to walk in without observing the niceties of knocking.
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Your young child calls plaintively from the next room. Alternately, your young child decides to walk in and then call plaintively.
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A sudden cramp announces the arrival of her period.
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She’s had too much to drink and is about to hurl.
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You know that playful little slap she really enjoyed 10 seconds ago? Well, that was then. . .
I’m sure other Dopers have experienced other situations.
I think this strongly depends on the situation.
Mid coitus with:
My long time SO and she sounds like shes in pain: Stop now.
My long time SO and she said it because she think she heard the 1 year old baby cry: Umm, let’s finish here.
My long time SO and she’s about to puke: Stop now
My, just started sleeping with her a few months ago GF and we’re trying out a new position that she’s not emotionally comfortable with: Maybe those few extra strokes, then stop and change positions.
I’m the ‘other guy’ and she just heard her husband pull into the drive way: Stopped before she even finished saying the word.
One night stand, she was iffy to begin with and she’s clearly changed her mind is is quite serious about it: Stop now. If we were both enjoying ourselves up to that point, maybe see if she’s willing to keep going, but wouldn’t push the subject.
One night stand and she said “stop” in such a way to imply that I was raping her: Stop now and GTFO.
By the way, I should add that I don’t see myself ending up in most of these situations, I’m just saying that the OP should clarify this a bit. Most of the poll answers could be ‘correct’ given the situation.
Holy cow, I had no idea there was so much coitus interruptus going on. While there were times I wished my partner would be done, I would have never considered being selfish enough to tell him to stop. I suppose if there were pain involved I would have a different tune but short of that, it’s just not polite for either person to orgasm and say cyakthxbye!
Then I’d assume whatever she was saying was ‘Oh God’ in Swedish, since I don’t speak that language.
What ? Is this a hunt down the rapists poll ?
If someone says “Stop, please get off of me” in the middle of sex, I figure there’s a damn good reason for it and I am removing myself from her body. She’s in pain or sick or she just realized she forgot her birth control pill or there’s a child standing behind me staring at my ass or she’s having an acid flashback and is about to stab me in the head or she just switched places with her alternate personality and while Tess consents Jessica does not.
If it was a “slower, slower!” or “ow, not that” or something, it’d be different. “Stop, please get off me” is pretty unambiguous.
Well, stop straightaway of course, but I’ll politely ask for an explanation after the fact.
I’ve just rolled onto my hairbrush!
But that’s ‘STOP!’ ‘Wait’ means ‘Mmm, right there.’ Luckily I think a monkey could understand the difference in tone and word choice (not that I swing that way).
Geez, I can’t imagine telling my boyfriend “stop, please get off” except for some of the above reasons (kids, roommates,pain or illness etc). If it’s been going on long enough to where things are getting uncomfortable down in girlbits land, I’d give him fair (sexily, gently and kindly) warning so that he could finish. I’ve never had a relationship (even with the few who were awful in bed), where this was an issue. (luckily the two who were terrible were also quick draw McGraw’s :D).
If I asked someone to stop mid way, I would have a pretty good reason to. Peeta Mellark you made me laugh. Yes, most of those that you mentioned would be damn good reasons to stop. These days, after a practically passionless marriage for almost 5 years (was married 16, got separated in August) I not only don’t ask my lover to stop, I beg for more …MORE! 
Crap. I didn’t realize your first post was the OP, I was basing everything on the question on the poll. Okay, so if she says ‘stop, please get off of me’, and she says it in English, I get off of her. Now the poll results make sense.