From Oct 20, 2014

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.