I think you are focusing on the wrong aspect of your proposed action. The question is not whether you owe it to P to accept their class arguments, the question is whether you owe it to P to be open about doing the deed anyway. While I think you do not have a more obligation to listen to P’s argument, having had this conversation with P, you are morally obligated to tell them ahead of time that you are going to do it anyway. Going behind their back and hoping they never find out seems like a betrayal of their trust / lie of omission. Having told in strong terms that they are against the action, P can reasonably expect you not do it behind their back.
I think you are focusing on the wrong aspect of your proposed action. The question is not whether you owe it to P to accept their class arguments, the question is whether you owe it to P to be open about doing the deed anyway. While I think you do not have a more obligation to listen to P’s argument, having had this conversation with P, you are morally obligated to tell them ahead of time that you are going to do it anyway. Going behind their back and hoping they never find out seems like a betrayal of their trust / lie of omission. Having told in strong terms that they are against the action, P can reasonably expect you not do it behind their back.