I didn't see the thread in question, but the scripture you question (1 Cor 7:36) states:
"If anyone thinks he is acting improperly toward the virgin he is engaged to, and if she is getting along in years and he feels he ought to marry, he should do as he wants. He is not sinning. They should get married."
In biblical terms, "marriage" is the act of becoming one flesh, sexual union through intercourse. As pointed out in this scripture, this is not to be taken casually or lightly, but after getting to know one another well enough (engagement) to commit a lifetime to the union of the man and the woman. One would be practicing fornication or adultery if after that union, they had intercourse with another who was not their mate.
The confusion comes from the modern day practice of "legal" marriage, which is actually not a scriptural requirement, as a Christian's "yes" should mean yes, and "no" means no. In contrast, in scriptural terms, once a female and male have sexual intercourse, they are scripturally "married". So it is possible to be legally married without being scripturally so, and vice-versa.
BA