It's the responsibility of the HCP to ensure that they have the patient's consent, including whether they are happy to be cared for by a nurse of the opposite sex. Never mind, 'she should have asked' - she should have been asked, and in such a way that she felt comfortable saying no if she did not want a male nurse.
Actually this is interesting. Because how do you ensure that you're asking in a way that ensures someone is comfortable saying no?
Obviously in this instance OP's aunt was uncomfortable. But we don't actually know if she was asked. If the nurse in question asked if he could help her to shower or remove her catheter and she acquiesced. Then that is consent given.
Obviously women (and people in general) can be anywhere on a scale between those that are feisty and assertive and those that are timid or have spent their lives being oppressed and find it easier just to agree and cope with the consequences. But in most circumstances a HCP - male or otherwise - has no way of knowing where on that scale the patient lies. So if they are asked, and they say yes they carry on.
Obviously you'd hope that had the aunt been crying or shaking with fear or otherwise clearly distressed he would have stopped and ensured that she was OK. But if she just let him get on with his job and complained to her niece later - how was he to know that her consent wasn't comfortably given?
Clearly what was needed in this case was for the aunt to be asked on admission (or another suitable time) for her preferences in terms of care and caregivers. But whether there is enough time or enough staff to feasibly do that is another matter.
As it stands the male nurse in question hasn't (as far as we know) done anything wrong, hopefully he doesn't want to intimidate or upset his patients. And now the OP knows she can advocate for her aunt and hopefully make sure she's not put in this position again.
I suppose the question is who ensures that vulnerable people without family receive the care they prefer? And what happens if all of the patients request a female or male nurse. At some point presumably there is a duty of care to ensure people are bathed, or that catheters are changed in a timely fashion.... what happens when staffing doesn't allow for this to be done?