I have read in a number of official places that divorce is not prohibited per se in Sikhism. Naturally Sikhi discourages divorce -- for all the reasons that every society and religious discourages the break-up of a family. Which religion encourages divorce? Some make it easier (Islam) and others make it impossible (Roman Catholocism you can only annul) if you want to retain your standing in the faith. None of them say -- Hey, divorce is a good idea!! Go for it!!
As for bigamy in Sikhism -- there are two ways to look at this.
When bigamy is permitted on the physical plane by law (borrowing from drkhalsa ji here) then a Sikh has a legal opening to have more than one wife. But today the Rehit Maryada explicitly forbids that. Will it come as a surprise to know that both Guru Arjan Dev and Guru Gobind Singh had two wives? And the Sikh maharjas had harems and they were Sikhs.
I don't think Balbir's question is about any of this. If we are the Soul Bride, as both men and women are, and He is our Husband Lord, then are married Sikhs practicing bigamy? To answer this question - refer to drkhalsa ji's comments above. Again it is a matter of keeping the world of material reality in focus as we move to the ruth of Waheguru.
Bigamy is not my most important question. Infatuation with the pleasures of Maya needs to be my greater concern. We cannot judge one another either. How many of us have prepared our bed to receive the Husband Lord? Who has pleased Him? Who has met Him and consummated this relationship? Only He knows the answer because he sees into each and every heart.