It's just a style issue. From 2004 until very recently gay couples in England could only get a 'civil partnership', not a marriage; possibly because of that 'X

It's just a style issue. From 2004 until very recently gay couples in England could only get a 'civil partnership', not a marriage; possibly because of that 'X is Y's partner' in the UK is these days understood as meaning a romantic attachment. (The business meaning still exists, but is phrased as e.g. "X and Y are both partners at Foobar LLP" rather than "X is Y's partner").