I don't believe that both people reach completion at the same time very often in the vanilla world. I'm not vanilla, so I don't know for sure, but from what I understand it's kind of a rarity in real life. Vanilla sex is usually written with a simultaneous finish, but I've seen a number of online conversations with people rolling their literary eyes about it being written that way so often.
In the world of BDSM, where submissives are often trained to wait until given permission to come, it should be a given that synchronized orgasms would be a lot more common. Shouldn't it?
I still see readers occasionally talking about it like it's purely fantasy though -- like it only happens in books. I've even seen people comment that they'd like to see it written more "real", where everyone doesn't finish at the same time. But in my world -- my real world, not my fantasy world -- it's the norm for people to come together for the big finish.
The other thing the BDSM world has going for it is foreplay. The majority of the time our sex has a long period of flogging, or nipple torture, or orgasm denial, or forced orgasm after orgasm after orgasm, or intricate bondage, or something that comes before the sex and either allows for many orgasms before the actual sex act starts, or denies those orgasms before the sex starts. Either way can work out just fine.
Sure, it's often a Dom's prerogative to just say, "Bend over" and then go at it with no foreplay... but that in and of itself, the fact that the submissive must do it, that she is being used with no care given as to how she feels about it at the time... that's usually a big enough turn on that foreplay isn't needed. Or, occasionally, the submissive isn't turned on at all, she just bends over and deals with it until he is done. It may not be a turn on when it is happening, but can be a huge turn on the next day when it is remembered.
But back to my point, the world of BDSM is, in a way, geared towards the ending being manageable, and thus geared towards simultaneous orgasms for the finish (assuming the couple isn't into long term orgasm denial). By the time the end is nearing the Dom will have either allowed the submissive to have already had many orgasms and can be building her up to the final big one... or he can have denied any orgasms for the past hour or more of titillation and then order it to happen just as he is about ready to finish. That is the norm in my world. It doesn't happen that way 100% of the time, but I would guess that the ending is within thirty seconds of each other greater than seventy-five percent of the time.... and at least ten percent of that other twenty-five percent is when he intends for it to not be simultaneous because he has a different ending in mind. Or that's what he tells me, anyway.
What about you? Do you see the whole simultaneous ending as fantasy or the way it usually works out in real life? Do you roll your eyes at a perfect ending? Would a book be more real for you if the occasional scene didn't end skillfully choreographed?