In Oriental Orthodox sacramental theology can a mixed marriage be valid? The Armenians and Syriac allow mixed marriages with trinitarian christians. But generally only the orthodox can take sacraments . How is such a mixed marriage valid then?
From The Sacrament of Holy Crowning in the Armenian Apostolic Orthodox Church

Today, this Eucharistic tradition is confronted with many challenges.
In our pluralistic society the great diversity of religious faith
confessions makes the “ideal” of a marriage between Armenian Orthodox
partners less a reality in the Church community. The Church now
encounters the condition of a “mixed marriage”; a marriage between an
Armenian Orthodox and a Christian from another faith confession. Such
situations do not change the general meaning of marriage with regard
to its intent but certainly present problems when celebrating the
rites of blessing.
It should be understood that mixed marriages are neither encouraged
nor forbidden by the Armenian Orthodox Church. Sacramental blessing by
the church granted through the person of the officient Priest could
only be administered and celebrated for those who are baptized
Christians. Canonically there is no separate liturgical rite for an
Armenian Orthodox and a non- Christian.
As stated, mixed marriages are those between Armenian Orthodox and
persons from Christian confessions that acknowledge belief in the Holy
Trinity and the Divinity of Jesus Christ as Lord and Saviour. The
celebration of the Sacrament of Crowning, as it is traditionally
called, is allowed for a mixed faith union out of concern for the
couple’s well being inviting them both to share in prayer and affirm,
however possible, their common faith within the rite without
compromise of personal or Church integrity. This practice of
“oekonomia” thus allows both to share as much as possible. It should
remain clear that the acceptance and participation of a non-Armenian
Orthodox Christian in the marriage rite does not imply his/her
conversion nor does it grant him/her the privilege to actively receive
the other Sacraments of the Church. It is only by the Sacrament of
Chrismation that one becomes a member of the faith community; after
proper catechizes before or after marriage.

Arguably, this is a palliative measure as a consequence of declining religiosity in Europe. The same phenomenon can be identified for other denominations in many European countries with low religiosity:

(Pew Research)
Which clearly reflects in the proportion of mixed marriages:

From Mixed Marriages in Europe, 1990-2010