Here is what some eighteenth-century dictionaries have to say about wife. From John Kersey & Edward Phillips, The New World of Words: or, Universal English Dictionary, sixth edition (1706):
Wife, a married Woman, whose Will, in the judgment of the Law, is subject to that of her Husband; so that 'tis commonly said She has no Will, but fulget radiis Mariti, i. e. Shines with her Husband's Lustre.
The same definition of A WIFE (but with an etymological derivation from the Saxon Wiff) appears in various editions of Nathan Bailey, An Universal Etymological English Dictionary (1724 and later).
From John Kersey, Dictionarium Anglo-Britannicum: or, a General English Dictionary (1708):
Wife, a married Woman.
From Thomas Dyche & William Pardon, A New General English Dictionary, first (?) edition (1735):
WIFE (S.) A woman that is married.
From Samuel Johnson, A Dictionary of the English Language (1756):
WIFE. s. plural wives. ... 1. A woman that has a husband. [Quotations from Shakespeare, Genesis, Milton, Dryden, and Pope omitted]. 2. It is used for a woman of low employment. [Quotation from Bacon involving the deceptive vending practices of "strawberry wives" omitted].
As Johnson's entry indicates, there was a strand of usage in England, at least from the time of Francis Bacon (who died in 1626), of using the term wife to refer to a woman of mean occupation. It is certainly possible that native English speakers of the eighteenth century might on occasion refer to a fishwife, who might or might not be married—and who certainly wasn't married to a fish—as simply a wife. (An alewife, on the other hand is a fish.) Nevertheless, to judge from the dictionary entries above, the word wife was generally understood in the 1700s to refer to a married woman.
On the other hand, nothing in any of the cited definitions suggests that the man in the nursery rhyme who was "with seven wives" had to have been married to them himself in order for them to qualify as wives he was with.