You’re right that the article reveals almost nothing about the book under discussion.

As to whether same sex marriage is equivalent to heterosexual marriage: It’s fairly clear, from Matthew 19 and other texts that, from a Biblical POV, it is not. But that shouldn’t be an issue WRT secular marriage, which is merely a matter of law — a contract, which needn’t concern anyone else.

The problem is, gays who want to marry also want to compel the labor of people who do not believe that their marriage is a “real” marriage in the religious sense, and who want no part of it. In states where LGBT folks are included in civil rights statutes, Christian business people who don’t want to serve same sex weddings have been subjected to financial ruin by gays and their lawyers.

That is why same sex marriage has become such a big deal among religious conservatives, and why they have been trying to pass religious freedom laws to protect themselves. If RFRA efforts fail, and they are failing because the economic oligarchy opposes them, Christians who are serious about Biblical teachings have to face the fact that secular society has become overtly hostile to them.

At that point, Biblical Christians have to look for ways to get out from under secular society.