What are these "other forms of union" that Francis speaks of? Are not all other sexual unions immoral in the eyes of the Church? Why can't the pope state this? Answer: because he doesn't believe it.
"It ... can no longer simply be said that all those in any 'irregular' situation are living in a state of mortal sin and are deprived of sanctifying grace," states the pontiff at one point in the document, released by the Vatican Friday."
"It is reductive simply to consider whether or not an individuals actions correspond to a general law or rule, because that is not enough to discern and ensure full fidelity to God in the concrete life of a human being," the pope writes later.
"Discernment must help to find possible ways of responding to God and growing in the midst of limits," states Francis. "By thinking that everything is black and white, we sometimes close off the way of grace and of growth, and discourage paths of sanctification which give glory to God."
https://www.ncronline.org/news/vatican/francis-exhortation-radical-shift-see-grace-imperfection-without-fearing-moral
Francis warm greeting a "married" homosexual couple.
This reminds me of the Vatican II theology of "full communion" vs. "partial communion" (pre-Vatican II it was in communion or no communion).
What Francis seems to be implying/inferring here is that, although these other unions aren't equal with marriage (ie. full idea) they are not completely invalid. This is very similar to the Vatican II belief that non-Catholic sects are means of salvation...that they have "partial" communion with the "full idea" (ie. the Catholic Church).