A scan of The Complete Works in Verse and Prose of Andrew Marvell: Verse. Andrew Marvell lived 1621-1678.
Marvell is saying that the eunuch will become famous through his poems, which will substitute for the children he will never have. The eunuch isn't "barren" insofar as he uses his "verse" to impregnate "fame," and his poems will be repeated ("Echo nurse..."), becoming "a tuneful race."
IN EUNUCHUM POETAM: Nec sterilem te crede, licet mulieribus exul / Falcem virgineae nequeas immittere messi, / Et nostro peccare modo. Tibi fama perenne / Praegnabit, rapiesque novem de monte sorores, / Et pariet modulos Echo repetita nepotes.
Translation. UPON A EUNUCH-POET. Deem not that thou art barren, though, forlorn, / Thou plunge no sickle in the virgin corn, / And, mateless, hast no part in our sweet curse. / Fame shall be ever pregnant by thy verse; / The vocal Sisters nine thou shalt embrace, / And Echo nurse thy words, a tuneful race.
NOTE. These Latin lines apeared originally in the folio of 1681 (p. 63), where they are headed 'Upon an Eunuch, a Poet. Fragment.' G.