You definitely have the right to be irritated, as that is the initial reaction of pretty much everyone. However, try your darnedest to see where the bride and groom are coming from. It’s actually quite standard for significant others of the wedding party to be excluded since “dating” is somewhat ambiguous and there’s the chance you might not be together by the time the Big Day rolls around. The guest list also likely has something to do with it–for example, there might be a significant number of people the bride and groom simply have to invite, such as relatives and people they’ve known their entire lives. If this is the case, they’re going to trim the guest list fat wherever they can. Budgetary constraints is a common wedding issue, and if it’s going to cost the bride and groom a lot of dough to invite the boyfriends and girlfriends of the entire wedding party, they’re not going to do it. Period.
It’s also important to remember that just because you aren’t invited doesn’t mean they don’t like you or you should take it as a slight. They probably want to get to know you as soon as possible, such as after the wedding craziness is over. Remember, the wedding isn’t about you–it’s about two people celebrating their love with those closest to them. Not getting an invite might sting a bit, but you shouldn’t take it personally. Be gracious, and if they mention they are sorry they can’t invite you, simply say you understand and that it’s just fine. They will like you all the more if you do!