How ethical is the practice of including spouses as co-authors when they are in the same field?
Your question is based on one assumption (that you spell out):  "the barrier for co-author contribution is far more easily passed [for spouses], compared to two regular collaborating PIs.".

"Yeah, well, you know, that’s just, like, your opinion, man." -- The Dude

As pointed out in a comment, ethical people don't draw the line in different places depending on the situation (at least for the case in question). The line is already drawn by the ethics principles they follow.

So, to answer your question, it is ethical to include spouses when the spouses contributed to the paper, it is not when they didn't. Exactly like including anyone else.