Since mostly the Titans indulge in marriage of brother and sister, it might be that Aeschylus is using an old tradition in which Themis is Iapetus' wife but that the Hesiodic tradition preferred that Themis and Mnemosyne be consorts of Zeus alone. But it would be been quite within Achaean practice for Zeus to have taken the wives of the Titans as his mistresses after throwing down their husbands.
Iapetus is sometimes equated by Creationists with Japheth, the son of Noah, based on the similarity of their names, though scholars of Indo-European linguistics dispute such an equation vehemently.