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When was this phrase put into the Temple ceremony?

Since all bigamy was illegal during Joseph Smiths time, it must not have been there then.
If not, when was it added?

By being there it takes away Polygamy/Plural Marriage completely if one is to follow its meaning. This includes all the US, Mexico and Canada. It takes away all excuse for more than one wife 'alive at the same time'.

If this really is believed and was part of the Temple ceremony during Josephs time or at any time during the active practice of Polygamy, doesn't that make all those doing so Adulterers?

That sounds like something that would have been added after 1904 or so, as a way of reinforcing the polygamy ban. I don't know this, however - I'm just speculating.

Actually, if an endowed Mormon were to marry a person of the same sex in some place where that was legal, they would be technically in compliance with the endowment covenant not to have sexual relations except with their spouse, to whom they are legally and lawfully married. I personally believe this conundrum contributes to the church's antipathy toward legal and lawful same-sex marriage. They don't want homosexual relations to be considered non-sinful under any conditions, yet if a same-sex couple were legally and lawfully married, there'd be technically nothing in the covenants a person had made (if endowed) to condemn them. The anti-homosexuality justifications would then have to be more explicitly made. I don't think they want to do that.

Right now they can stand back and justify the condemnation of gay sex on the grounds that, being unmarried, the partners are committing fornication. If they are married, then they have to condemn it explicitly because it's two dudes, or two girls, notwithstanding legal and lawful marriage.