There is a distinction. For sure, when an Orthodox or a Catholic is '"married" in a non sacramental manner by some ecclesial community' (or by some state functionary) it is not a marriage and should the question arise, the Church will annul the union.
However, a homosexual couple that is "married" under some legal construct cannot even ask for annulment.
In the first case the Church would presume a validity and working from that assumption will discover nullity. If the purported marriage was not sacramental, the discovery process would complete almost as soon as it started, but still it is a process of discovery that starts with presumption of validity. For example, if such spouse does not ask for annulment, the Church views them as married couple. If the canonical invalidity of their marriage comes to light, the Church will encourage them to consecrate their marriage. I don't think they will even be asked to separate for a while (correct me if I am wrong).
No such presumption can be made about a homosexual couple.
That is, of course, in complete accordance with the natural law. When a man fornicates with a woman, their sin is in yielding to a natural impulse, which is in itself healthy. When a homosexual act is committed, the sin is in the homosexual appetite itself, and secondarily in the lack of self-control. It is therefore a greater sin because it is a compounded sin.
We do not annul non sacramental unions. Let’s say a couple of protestants, “married” by the Rev. Billy Bob Jeff and his snake, want to become Orthodox. Among other things, it is likely that a period of continence and confession will be required followed by a sacramental marriage or perhaps a sacramental blessing of the existing union. At that point, the now Orthodox couple can be admitted to communion.