In Catholic dogma, you have something called grave matter. But for sin itself, you need three elements. Grave matter is the "tinder" of sin, so to speak, but not sin itself.
"three conditions must exist at the same time.
It must be of a grave matter;
It must be committed with full knowledge that it is a mortal sin;
It must be committed with full consent. [ Full consent means to do it "voluntarily."]" ( C.C.C. # 1857)
In the concrete circumstances of a person's life, the need for companionship and to express sexual urges that are innate to a person, would likely mean that for many or even most people, that's not the case. Only God determines that and an individual person forms their conscience in accordance with what it tells them and by respecting, if they are Catholic, revealed truth as earnestly as possible and heeding the guidance of their pastor, the priest.
Now, an individual homosexual relationship could, for instance, have more perfect qualities and be closer to the church's ideal of marriage than a bad heterosexual one that involves abuse, unfaithfulness or anything else that offends the sanctity of marital love.
Those elements of holiness and truth are real in that relationship, implanted by the Holy Spirit, and could help the same-sex partners grow in holiness as they "form their own consciences" and work out their salvation with, rather than against, the pastoral guidance of the Church.
Just as you can be baptized by implicit desire in complex situations that don't permit actual baptism, such as being raised in a different faith or sincerely convinced that another religious path is true for innumerable reasons that excuse, so to can a same-sex union be implicitly oriented towards the kind of grace conferred by marriage, without ever being so in the Church's eyes.
Even though the church cannot for sacramental reasons, requiring as per Ephesians a female phenotype and a male phenotype in sacramental sex (modelling Church as Body of Christ and Christ) doctrinally and liturgically recognise any union as a sacramental marriage except that type of one, other unions - such as heterosexual cohabitation or what the Church traditionally called concubinage and tolerated in the first millennium or indeed same-sex civil unions - can still have elements of holiness and truth that lead the partners on their path to salvation.
They can still have saving grace, even in a situation that's objectively not in line with the church's doctrinal standard. The parameters of this were all laid out back in 2016 in Francis's apostolic exhortation Amoris Laetitia. So this is just a final "working out" of the groundwork already set, pastorally, in that document by the papal magisterium.
What's clear is that as this papacy reaches it's maturity, after a decade on the Chair of St. Peter, things are definitely crystallising in advance of the handover to the next pontificate.
To cut a long story short, the new pastoral guidelines are good and doctrinally sound (of course!), perfectly consonant with the church's sacred tradition and are what I've been hoping would come out for a long time. Of course a pastoral blessing should be possible.