Using birth control is not obviously equal to zera levatala, but I think your statement about Rabbeinu Tam is assuming that (if I'm understanding you correctly). || Does the Tosfos HaRosh then hold it is Rabbinic (because ולערב would seem to be)?

@Yishai the Rishonim are assuming that. They are discussing the gemara about birth control.

Now though that there is a rabbinic (or whatever you want to call it) prohibition on polygamy, one can argue this wouldn't be delaying it in any way. Same with one who has a Heter to not be married at the age of 13 (most of us). The obligation may be latent, but this isn't nullifying it.

Not seeing how you see that. Nothing you quote mentions it.

@Yishai משמשות במוך?

@Yishai re the Rabbinic suggestion, I can't prove it, but such a distinction is not mentioned anywhere. And, the Behag in Hilchos Kesubos seems to understand that continuing to have children is a kiyum of peru u'revu. I think they understand ולערב to be qualifying peru u'revu, not an independent mitzvah.

@yEz, that is birth control, not זרע לבטלה din.org.il/2013/02/02/…

@Yishai See Tosefos Yevamos 12b s.v. שלש נשים. Note that he is citing Rabbeinu Tam

@yEz, but how do you know he is not talking in Sefer HaYashar about אחר התשמיש?

@Yishai because then R"T says it isn't assur... R"T is explaining how you might have said it is assur.

@yEz, you might have said it is assur because it is מבטל פריה ורביה, perhaps of her husband. As far as I can tell, the equation of that to זרע לבטלה is what you read into it.

@Yishai so then why isn't that the issur under discussion in Yevamos? It's the same ביטול פריה ורביה - why mention hashchasas zera?

@yEz, because he reads it as אחר התשמיש. That is the whole point in Yevamos - קודם is always assur, even in the three circumstances, after is required in the three circumstances but always allowed otherwise. He is saying that one who is commanded in פריה ורביה is commanded in השחתת זרע

@Yishai I am not going to argue the point further, but I still disagree.

@yEz ממה נפשך: if you are still obligated to have kids after you had kids, then contraceptive is asur, and if you don't remarry after the wife reaches age 50, youre over the issur of zera levatala even when youre with her, since you should be with a new wife. Or, if its not a chiuv to keep having kids, then it should be mutar in any case you are אנוס or already done your part

@DoubleAA Seems logical, but it bears mentioning that all this is within the framework of p'ru urvu and the opinions of Rabbeinu Tam and the Tosafos HaRosh. Of course, there are other opinions that give different reasons for the prohibition (according to which it would still be prohibited).

@Fred would love to hear

@yEz Tosafos in Yevamos says in the name of Rabbeinu Tam that Kodem Tashmish would never be allowed. Your quote is very obviously talking about Le'achar. That is why the whole issue is of פריה ורביה. The Gemara in Niddah also cleary differentiates between marrying someone young, unable to have children, and Mashchis which is Mabul, Misah and like murder.

@Fred it also bears mentioning that I selected out two random Rishonim, but the Behag, Ramban as explained by Ritva, and many other Rishonim use the same framework. Also, the suggestion made by DoubleAA is a possibility but certainly not a logical consequence of this approach - the fact that one may be currently unable to fulfill a miztvah does not mean he is not obligated in that mitzvah, nor does circumstance which is created and alterable by the involved party necessarily create onus, even if it amounts to an exemption.

@yEz Fair enough. The mechanics of a bitul asei deserve a proper treatment of their own, particularly the question of whether an absolute but non-inherent prohibition preventing someone from fulfilling a mitzva (e.g. polygamy with respect to p'ru urvu) renders the possibility of a bitul asei moot.

@yEz Further, the mechanics of bitul asei vis-a-vis השחתת זרע require clarification (according to those opinions who see that as the problem): Is the destruction of any particular זרע considered a bitul asei since you are preventing it from being mazria', or is it simply a matter of whether an individual will more broadly be prevented from fulfilling p'ru urvu? If the latter, then why is this viewed as an absolute prohibition according to opinions that view the prohibition as a bitul of p'ru urvu (since, for one thing, the action doesn't always prevent p'ru urvu)?

I couldn't find a Tosafos Harosh on Nida. It didn't seem to be part of the set. But I did see the Tosafos Harosh on Yevamos 12b which is very explicit that קודם תשמיש would never be allowed according to Rabbeinu Tam.

@HaLeiVi Tos. HaRosh is on the side of the page in the gemara