Walter Trobisch, in his book I Married You, sees marriage as having three aspects: Love, one flesh (sex), and wedlock. The problem with a “trial marriage” is that it isn’t really a trial, since it is undertaken outside of one of the key aspects of marriage, the permanent bond.

[It] is one of the greatest temptations of our time: to consider the legal act of the wedding as a mere formality, as an unimportant piece of paper which one can get someday, or maybe not at all. One pretends that the two angles of love and sex represent the whole of marriage. “Some people, in all seriousness, propose trial marriages. They suggest that a couple live together for awhile in order to see whether they fit together. If then they come to the conclusion that they do not, they can separate without risking a divorce. But the whole proposition rests upon the illusion that the two angles of sex and love represent the whole. Since they do not, marriage cannot be tested in that way.