Thanks for your reply, Janx! Our brew was 6 lbs of extract and 1.25 lbs of various grains. As for the taste, I wouldn't call it sweet. I just remember the taste I encountered when I snuck a little taste during bottling, and it was just like that...bitter, as though I had chewed a couple grains and taken a mouthful of water. You mention a number of potential causes, and I suspect we went wrong with a number of them. For example, we didn't monitor the temperature of our boil, so our mash may have been too hot. And I strained the mash using a kitchen strainer that may not have been fine enough, so I think there was some degree of grain in the fermentor, even though it looked to me like I got most of it. Our brew is now two months old, so as a porter, it may mellow a bit with some additional ageing.
Based on what I've written, I'd be grateful for any other advice you have. Thanks again for your reply.