"Neither Congress nor the States shall make any law recognizing, defining, or granting any special status or privileges to any person based on marriage or marital status. All such laws currently in existence shall be updated to comply with this amendment by 180 days after its enactment.
Congress and the States may, if they so choose, make laws defining a new status and granting that status to persons married at any time prior to 180 days after the enactment of this amendment. This is the only purpose for which marriage may be recognized in any way by any law. No law may recognize in any way any marriage occuring more than 180 days after the enactment of this amendment.
Neither Congress nor the States shall produce any laws or documentation using any of the following terms related to marriage, EXCEPT for the purpose of carrying out the laws permitted in the previous paragraph: marriage, married, marital status, spouse, husband, wife, wedding, bride, groom."
This would essentially require the government to either get out of the business of marriage altogether, or define "civil unions" for heterosexual couples and grandfather existing married couples into that status.
The trick with this amendment is that it doesn't say anything whatsoever about what the government can or can't permit or prohibit for gay couples. Based on only this amendment, the government would be entirely free to define the new status as "Heterosexual Union" and refuse to ever grant it to gay couples. The trick is that by forcing the government to define an entirely new term for everyone, rather than reusing a loaded word like "marriage", it's far easier to apply the common sense interpretation of the rest of the constitution to any law they make.
Non-governmental and religious institutions would of course continue to be free to talk about marriage if they wanted to, and could apply whatever restrictions they liked to the meaning of it.