To think this is a peculiar/sexist assumption?

[lessonsintightropes]

Not a biggie really. Just going through the bumf on my new pension, which assumes that the beneficiary is my husband, who is three years older than me.

Now as it happens I do have a DH, and it says in small print that it could also refer to my husband/civil partner/other dependent, and that the 3 years older is 'assumed'. It just made me on both counts, and would have been unimpressed if single/in a same-sex relationship.

Is it just an example situation to match a stereotypical relationship?

They just go on the average, which is why they call it an 'assumption' and expect you to notify them if your situation is significantly different. It's like any kind of insurance where they assume certain things about you in order to simplify the process.

This will be a computer generated projection which uses a standard template to calculate.
These things are generally sent out at the start of a policy and annually. As you come close to retirement age, a quote specific to your circumstances will be sent out as standard.
You can also request a personalised quote at any time.
It wouldn't be cost effective for a company to check all circumstances of all customers each time they send out a general projection.

This is especially important with a pension since it runs for such a long time. It is likely your circumstances will change between start of policy and retirement age.

It seems you're looking quite hard to find something offensive, even if it isn't personally offensive to you.