Parsing, I agree with everything you just said, and finally feel like this conversation is moving in the right direction!
It could be, that banks (or more specifically, bank systems or bank tellers) are making assumptions that the male should be the primary account holder. This, would be a feminist issue.
It could be, that when a mixed sex couple are applying for something, they are putting the man first, because one or both of them believe the man should be first (or could be the main earner, or is in charge of finances). This, would be an example of patriarchy.
It could be, that when a mixed sex couple are applying for something, they are putting the woman first, because the woman is in control of the household finances, or the main earner. This, would be an example of feminism achieving its goal.
It could be that joint accounts just aren't 'joint' in the sense we expect, and maybe banks should rename the accounts. Or maybe they should look into how joint accounts work, and ensure they are, indeed, absolutely joint.
But to say this is outright a feminist issue, with absolutely no real knowledge of any of the above, is trite.
If the patriarchal examples are true, then I absolutely agree, this is an issue that needs to be addressed. But if it is simply an issue of one party, of either sex, receiving detrimental treatment, well that's just the way joint accounts work. If you don't like it, don't have one. They aren't exclusively for mixed sex couples, so this issue will always exist (unless the banks rethink they way joint accounts work).
But the fact my boss gets paid more than me, doesn't make it a feminist issue, just because he is a male and I am a female. There are plenty of men who work below me. I worry sometimes that attributing feminism to every single issue without it being directly affecting women and women only, breeds the opinion that feminists are all radical, and belittles the root cause of the feminist movement.