I have several friends who have significant others, and, of course, I will include them by name on the invitations. That's why I was dismayed and even offended when I recently received an invitation to a friend's wedding that did not include my fiance. Granted, my friends who are getting married don't know my fiance well, but they certainly do know of his existence. I feel funny about responding for two when only one was invited, but I also feel funny attending by myself. I know my fiance would be hurt if I went without him. How do I handle this? If nothing else, it's a good lesson for me when I start sending out my wedding invitations!
Unfortunately, you can't RSVP for both of you, because only you are invited. It's great that you are inviting all of your friends with significant others, but you can't dictate how someone else puts together their guest list. Technically, yes, engaged guests should be invited with their future spouses, but sometimes (as you may well know from your own experience) couples have to make tough list-cutting decisions. They want all the people they are closest to there on their wedding day, and that sometimes means leaving other people out. Obviously you are important to them, because they invited you; perhaps they didn't invite your fiance because space (and money) was tight and because they don't know him well. Try to look at it that way: it most likely wasn't an intentional slight. Now you must decide whether you want to attend the wedding solo or not. You could always just send a gift if you're uncomfortable going alone.
See More: Guests & Guest List , Going to a Wedding? , Wedding Guests