Re: love as a basis for marriage--it's true that it's impossible to pinpoint any one century as 'the' time when that became a universal standard in our culture. Historians generally rely heavily on sources like spousal litigation records from ecclesiastical courts for information about non-elite marriage in the pre-modern period, and what those sources seem to indicate is a gradual progression from consent (mostly to arranged marriages) as the basis for marriage in the High Middle Ages (11th-13th cen.), to love as the basis for marriage by around the 17th century (except among the wealthy and powerful, who often continued to practice arranged marriage). If you have access to an academic database, it's not difficult to quickly locate a generous handful of scholarly articles on this topic. Love poetry is highly unreliable as historical source material, since it's not intended to accurately portray the practical application of social customs; I'd point to India as a particularly clear-cut example of that--arranged marriage unambiguously was and is the norm there, yet love poetry has just as long a pedigree in their major literary languages as it does in ours, and romantic love, while in practice traditionally seen as self-indulgent-bordering-on-sleazy, is in literature (and for that matter, Bollywood) constantly idealized, as exhilarating emotional states often are in art and literature the world over.
Also, Re: polygamy in Roman-era Judaism and early Christianity--while polygamy wasn't actually banned in Roman law until 212 AD, yes, it definitely wasn't a Greco-Roman custom (though concubinage certainly was); and, yes, this undoubtedly had a strong influence on both Jewish and Christian views of polygamy, both at the time and in subsequent Christian history. The 1st-century Jewish historian Josephus noted the practice of polygamy among the Herodian kings and the very wealthy, but seems to treat it almost as a curiosity, despite his awareness of its long history among the Jews; and the Mishnah and the Dead Sea Scrolls suggest that both the Hillelite Pharisees and the Essenes opposed polygamy, quite possibly largely due to contemporary Roman influence.
I am still really having trouble understanding how the shifts to romantic love as the basis for freely choosing one's spouse, and to equal rights for women, DON'T constitute 'core' redefinitions of marriage. As Irvine just touched on, the 'penis + vagina definition' obviously refers to marriage in its aspect as a means of formally continuing kinship outside bloodlines--but mutual personal fulfillment is just as obviously central to our present definition...as our expectations of choice in spouse, right of the woman to work outside the home, and right to decide whether and when to have children together all clearly show. Unless one's intent is to restrict marriage solely to couples who are A) physically capable of conceiving a child together and B) willing to take some sort of oath of intent to do so, then it really makes no sense to insist on 'penis + vagina' as 'core' to the definition. Granted, if our social reference point were Israel 3000 years ago, then a case could be made that there's no double standard--but only because adult life for almost everyone then meant: you marry the (wo)man your parents choose, conceive and raise numerous children together, and if you don't much like your spouse or your 'job', well, too bad for you; suck it up, you're hardly the only one unhappy. Marriage (and parenting) will never be nonstop bliss, human relationships seldom are; but fortunately for us we live in a time and place where those duties can be freely chosen and shared, and grounded in joy and gratitude for having found each other.