Answer:
Rabbein Asher, followed by Rabbi Elijah Spira, commented that an occasional exterior ejaculation in tandem with ones wife is not considered “extracting semen in vain” (and not banned by the talmud) as long as the intention is not to avoid impregnating one’s wife and it is done on rare occasion -as this is not likened to the desire of Onan who wished to avoid impregnating Tamar entirely.

The Aguddah work also sides with the lenient opinion permitting an occasional extra-vaginal ejaculation with one’s wife, whilst Rabbi Samuel Eidels (the Maharsha) likewise taking a lenient view.

Rabbi Eleazar of Worms, in his recently published Torah commentary to the verse “Adam and his wife were naked, and where not embarrassed” (genesis) permits any activity with one’s wife necessary to “quiet (lit. seat)” his desire.

Rabbinic authorities have in certain instances permitted intentional extra-vaginal ejaculation in tandem with one’s wife. The tannaim Rabbi Eliezer and Rabbi Meir (Tosefta, Niddah 2:), for safety reasons (prior to the advent of female birth control) permits exterior ejaculation for a duration of 24 months post childbirth (Talmud Yebamoth 34).