Information that men consider carefully includes phone and broadband agreements, as well as car insurance policies.
Commenting on the findings, Paul Baxter of Tescocompare.com said: "It is not surprising that so many people find themselves in a situation where they thought they were covered when in fact they weren't.
"The predication fuelled by many comparison sites to simply purchase car insurance on price alone perhaps contributes to this lack of understanding."
Previous research by Tesco also found that co-habiting couples can save around £100 on their car insurance premiums by adding their partner as an additional driver on their policies.
The same study found that two-thirds of couples do not inform their insurer when they move in together.
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