It is widely known that wasting seed is one of the big sins a person can commit.
However, it seems that having relations when conception is not possible--such as during menopause or during pregnancy--is perfectly fine and is even sometimes recommended.
What exactly is the difference and why is the latter permitted?
Wasted means for no Halachicly recognized purpose, not necessarily done for some reason other than the purpose of conception. Regular relations with ones wife is an obligation, and thus serves a purpose, as does having children.
There is also some discussion of the specific method having to be כדרך הארץ - the natural way of marital relations. This impacts some discussions around birth control as well, so it isn't enough that it just be for the purpose, the method by which that purpose is accomplished is also relevant.
Sources. (שו”ע אבה”ע סי’ כג סעי’ א’: “אסור להוציא שכבת זרע לבטלה ועון זה חמור מכל עבירות שבתורה”. רמ”א שם סעי’ ה’: “אבל מותר לשמש עם קטנה ואיילונית, הואיל ומשמש כדרך הארץ (תוספות ומרדכי פ”ק דיבמות ונ”י פרק הבא על יבמתו)”.
ראה שו”ת אגרות משה אבה”ע ח”א סי’ סג ענף ג’: “אבל לפ”ז ניחא דכיון דפירוש לבטלה הוא לבטלה ממש שאין בה שום צורך ומטעם זה מותר לצורך מצות עונה ושמחת אשתו”. ראה עוד שם)
Reb Moshe Feinstein in his tshuvos about i.v.f. separates the issue of niuf byad and hotzaas zera. And whereas hotzaas zera is allowed for a productive purpose such as having a child, niyuf biyad is never allowed. So cohabiting in a normal fashion, but collecting the sperm in order to impregnate his wife with it is allowed. Masturbating into a cup for the same purpose is not allowed.
I'd just add a Drush explanation in the דרך העבודה approach:
Explanation 1:
Wasting means there is no reception כלי. A woman is a כלי as the Gemmorah says עשאה כלי. In any way of copulation (vag' and non-vag') the woman is considered a כלי and therefore allowed (see Gemmorah in "חתיכה שהביא מהשוק").
When there's no כלי - the זרע goes to חיצונים and therefore prohibited.
Explanation 2:
יחוד has two purposes - 1: procreation (פרו ורבו) and 2: יחוד by itself (the FUN). There are different כוונות of יחוד for Shabbos and weekdays.
That's the Gemmorah in Kesubos 59b: "אין אשה אלא לבנים" - procreation, "אין אשה אלא ליופי" - יחוד itself.
THe Poskim focused only on procreation, saying that even while pregnant the fetus benefits etc. But the truth is those relations are not only permitted but obligatory as procreational while focusing on the יחוד of the couple itself.
