"If the Ree (Yevamos 34b) would have only seen what the Zohar says about spilling seed for no good reason - that it's the biggest sin in the Torah, he never would have said what he said" (That you're allowed to do anal sex all the way till the end) (Bedek Habayis Ev He 25)
The thing is that even if the Ree did not see the monster of the Zohar he definitely saw the monster of Needa 13b, so the question is, how much difference is there between the monster of Needa 13, and the monster of the Zohar, since the Poskim say anyway that the Zohar is not accurate, and it's really not the biggest sin in the Torah.
Another question is that since the Ree said what he said based on Needa 13b, if you're gonna tell me that the Zohar would change the Ree's mind then the Zohar is basically knocking out the Talmud, and we know that when the Zohar goes against the Talmud we do not listen to the Zohar. It's not for nothing that the Talmud was sealed shut, and from now on we try to understand the Talmud, not argue with it.
And anyway, I don't really see how the Zohar is a bigger monster then Needa 13 like I said, and I don't understand why in the world it's not OK to let a Jew sin a big monster, but it's OK to let a Jew sin a small monster, like Reb Moshe asks in Ev He 1 page 155.
The bottom line is what Tosfos says, and what Reb Moshe says, that the sin of spilling seed is only for no good reason, but anal sex is a good reason, because since you "buy" your wife she is yours, and you're allowed to eat her any way you want, and this is part of marriage. (See Jul 26, 2012) and since anal sex is a legitimate form of sex (If you do anal sex with a man or animal you get killed) therefore you have a right to do full blown anal sex for the enjoyment of marriage, even though the wife does not really enjoy anal sex and you have no bonding, like I showed you in Jul 31, 2011. (See Egros Moshe Ev He 1 page 155/156, and see May 9, 2013)
And now you know why Er and Onan did anal sex instead of pulling out the last minute, because they knew good and well that it's OK to spill the seed with anal sex. However, since they ONLY did anal sex it was quite obvious that they did not want Tamar to become pregnant, and that's why G-d killed them. And that's why the heter to do anal sex is only once in a while.
Is the Torah of the Reshonim "shaky" , that if they had only seen the Zohar they would have said differently? See Apr 1, 2014 how big the Ree was.