I have to say that regardless of whether or not you talked about it previously, he needed to tell you that this was something he planned on doing. Why? Because every time he engages someone else sexually, he is also risking your health.
Quite frankly, I wouldn't be able to trust someone who had done this. Don't get me wrong, it's not because of what he wants or that he's posting online or any of that. It's because he didn't allow me the chance to accept or reject the risk he's taking with my health.