The legislature can do whatever it wishes regarding same-sex marriage, but if a future referendum is held using this wording it will be easily defeated.
Most people are amenable to the idea that if two people of the same sex are truly in love and committed to each other the state should afford them the benefits of marriage, such as rights of survivorship and related financial and legal protections.
This would be called what it is --- a civil union, protected by civil rights. But we do not have either the right or the power to call something whatever we want. Words are important markers that point to a deeper reality which we may not fully understand.
Defining same-sex marriage as civil unions might be more acceptable to conservative churches and other religious institutions that oppose gay marriage on the grounds that marriage was first instituted and defined by God in the union of Adam and Eve. But even this verbal concession would not erase many people's justified concerns about the effects of the change on society. Civil unions are within the purview of the government to authorize only as long as there are no legal ramifications for circumstances in which they conflict with traditional values or established religious beliefs or practices.
Marriage as understood through the centuries is not innately a civil right, but it has been mistakenly thought to be one because of its universal acceptance. Its definition, as well as its substance, emerges from historical, religious, and cultural definitions that cannot be amended by legislative fiat.
The government is showing its political hubris in assuming it is acceptable to re-define marriage, whose definition has achieved its current pliability only because it is no longer taken seriously as a life-long commitment in our society. This breakdown of commitment is what has opened the current discussion and controversy.