If the Altarians are attacked, all of the other good civs will come to their aid. This is due to their super-ability, called "Super Organizer". Since you have (evidently) chosen good as your alignment, you are also being asked to declare war. You can refuse, and, other than a *possible* hit to your diplomatic relations with the Altarians, will suffer no ill effects.
There is no way that you can be forced to declare war against your will. There is, however, nothing to keep the AI from declaring war on you. Are you sure that this isn't what's happening? Pay attention to the GNN next time. When you declare war, it will read something like "The Terrans have declared war on race x". If the AI declares war on you, it will, of course, read the other way 'round.
I have seen this event many times, while playing as various races; I have always declined, and have never experienced what you are describing. I have never, in TA at least, played the Terrans as good, so this may be tied in to that particular scenario. If so, then this is a bug. Most likely you are missing or maybe mis-seeing something. Until someone else comes along that has seen this, I would just try and figure things out at your end.