An answer here states:
The mitzva of Pru Urvu covers only marrying someone believed to be fertile.
And one here:
"The mitzva per se of pru urvu means: "marry someone who is able to bear children, to the best of your knowledge, and go about normal married life."
I had thought that p'ru urvu simply meant having children - -a boy and a girl, or something like that. But apparently it actually [only] means getting married to somebody with childbearing potential.
Does doing so fulfill the actual mitzvah of p'ru urvu? That is, if one marries someone presumably capable of having children, and then does not/can not have children, has one still fulfilled the mitzvah of p'ru urvu? (I know that a remarriage is no longer halachically required. But do we consider that he did fulfill p'ru urvu, or that he "couldn't"?)
1b. If yes to the above: How long does he have to remain married to the woman (and/or live with her) during her presumed-fertile years in order to be yotzei the mitzvah? ("Until she has children" would be a very unsatisfying answer.)
If one does not marry a woman presumed to be fertile, but has [a boy and a girl] anyway--i.e., with one's presumed-infertile wife, through relations with a non-Jewish woman or non-wife, or through sperm donation--has one fulfilled p'ru urvu? Would these four cases differ?
In light of all the considerations above, is there a "cutoff" for p'ru urvu, insofar as it may become increasingly difficult with age to attract, marry, and get along with a younger woman -- and as communities have various standards about acceptable age difference?