Hindu marriage today is subject to, and governed by the Hindu Marriages Act, 1955 and its amendments by Parliament. This statute talks only of solemnizing a marriage; it does not make any reference to the "sacredness" of the ritual. It talks about completion of the marriage where the saptapadī is included in the customary rites. (For those hindus who do not have saptapadī as a part of their customary marriage ceremony, this does not apply.)
Now, in the case of tabras, the saptapadī is a part of their customary rites of marriage, but it consists of the groom taking the right toe of the bride with his fingers and making her take seven steps by the right side of the sacrificial fire, himself chanting the seven mantras starting with ekamiṣe viṣṇustānvetu etc. However, the law states that "Where such rites and ceremonies include the saptapadi (that is, the taking of seven steps by the bridegroom and the bride jointly before the sacred fire), the marriage becomes complete and binding when the seventh step is taken." (sec. 7(2) of the Act). Therefore, I feel our tabra marriages may, strictly according to the Law, be all null and void ! Hence, any talk about such invalid marriages being a sacred relationships loses its validity AFA tabra marriages are concerned, imo.
Another aspect of the hindu marriage is that according to our Dharmasastras (which governed the hindu marriages before the Hindu Code Bill was passed into Law by the Parliament, and which the Kanchi seer Chandrasekharendra Sarasvati himself wanted to be continued), there are eight types of marriages : brāhma, prājāpatya, ārṣa, daiva, gāndharva, āsura, rākṣasa and paiśāca. In the first four types, it was expexted that a girl should be a nāgnikā (one who looks pretty even without clothes, one who is nearing puberty, etc.,) at the time of marriage.
We have come a long way from all those beliefs and today, the tabra marriage is neither according to the Dharmasastras nor according to the Hindu Marriages Act, strictly speaking. Why then should we consider that marriage has still some "sacredness" attached to it? Is it because the males want to restrict the freedom of the woman for divorcing, by ascribing some kind of sacredness to a purely contractual but solemn agreement between a man and woman?
Yet another dimension about hindu marriages is what Atharva veda 5-17-8 & 9, says:
8. And if [there were] ten former husbands of a woman, not Brahmans,
provided a Brahman has seized her hand, he is alone her husband.
9. A Brahman [is] indeed her husband, not a noble (rājanya), not a
Vaiśya : this the sun goes proclaiming to the five races of men (mānava).