I can't explain why it was placed there. As I read this Pastor Immanuel, I see the current wording to be pointed only to the marriage relationship, that is adultery. There is no adultery outside of marriage. Immoral sexual practices may be disciplined, but it is not adultery. To place this language in the section on adultery is not correct. Immorality outside of marriage is not Bible adultery. And, within marriage, pornography is not a violation of the seventh commandment which is adultery and adultery only. Thus, I see a lowering of the marriage standard. One may walk away from a marriage if the spouse views a naked person one time. This is most serious. I did not agree with Daniel Jackson. The proposal was clear about what a marriage is. But, because he objected, the motion t.o change did not pass. Otherwise I believe it would have. I may see this all wrong, but this is how I read it. Another attack on marriage.
Back to your concern. In another section not having to do with adultery, I can see that including the terminology "legal binding marriage" is important for the reasons you stated. Sadly, this was not explained by the church manual committee, neither was it brought up by any of the delegates after Jackson objected. It was all very sad. It seems to me that this business meeting is not going as smoothly as previous Sessions. Yes, there is a division in the church, but when the motion to return made by Jackson was voted on, I was shocked to see it passed. I understand the concern about the failure of society, buy to follow the motion made by Jackson really did surprise me, both because of who made it and because of the lack of the need to change it. And, if the reason for the insertion of "lawful binding" was as you say, it makes perfect sense to include it. To then exclude it would leave out the concern which is not only legitimate, but needed.
But, from all I see, it does not belong in that section (adultery) in the church manual. You have the manual in front of you. Is may concern valid? Is this subject being misplaced in the manual, and will it weaken our position on marriage?