The swinging single does not want to be tied down, but rather to come and go as he wishes. Medieval Registers, in fact, referenced this happy-go-lucky bachelor by the Latin solutus, "loose or unchained," making him footloose, "unfettered," and fancy free, (early 20thC.), "one with nary a romantic commitment."
But as fate (what else?) would have it, he chanced upon this ravishing woman who managed to absolutely enthrall him. Thrall is Anglo Saxon for "slave." When you enthrall another, you "reduce him to the condition of a thrall" — in short, you enslave him. Soon he was — what else — but a "prisoner of love."
Before he knew it, he agreed to tie the knot (11th C.), further underscoring the nature of this prospective entanglement.
But what's a rope when you have wedlock?
To Lord Byron "A wedlock and a padlock mean the same thing." He had to be kidding. The Old English weddian was a "pledge," and the lock was actually a lac or "gift," our first wedlock being the present given at the time the pledge of marriage was made, helping seal the engagement.
After years of marriage, things may end up deadlocked (c.1778), "at a complete standstill," a term that preceded the "springless lock that opens with a key." There is no guarantee, however that the key to resolving the outstanding issues will ever be found, thus ensuring closure.
According to your local bookie, a lock (c.1940s) is a "sure thing", an "absolute certainty." Marriage is many things, but a lock? Fuggetaboutit!