Bahá'u'lláh was a polygynist, having had three wives. Yes, they were concurrent, so far as I can tell. Some Bahá'í apologists have attempted to justify his practice of polygyny before the western world by explaining that Bahá'u'lláh married his second wife, a cousin of his, to protect her, and that he married his third wife because Shi'ah law required that a man make his female servants to be his wives. I suppose this would have made his third wife a concubine.
None of these justifications explain why Bahá'u'lláh proceded to sire seven children between them. Did he visit their beds to do them honor?
I think the most dignified treatment of this issue is to simply acknowledge that Bahá'u'lláh was a polygynist, and point out that this was not uncommon for Muslim men of status in the 19th Century. To claim, however, that Bahá'u'lláh was not, somehow, a polygynist at heart, is to ignore the obvious. The Bahá'í Faith did not embrace monogamy and the gender equality implied therein until `Abdu'l-Bahá made it so.
`Abdu'l-Bahá did not only change Bahá'u'lláh's marriage law. He also effectively annulled the families of Bahá'u'lláh's 2nd and 3rd wives as well – by means of excommunication, while leaving his family untouched. Their excommunication would be left to his grandson and preferred heir Shoghi Rabbání. `Abdu'l-Bahá's you're with me or you're against me policy was embraced by his grandson, and the price they both paid for their unwillingness to condone the slightest dissent, even when it was motivated by a person's faithfullness to their immediate family, was the annihilation of the Aghsán line that Bahá'u'lláh based his covenant upon.