I'm going to be rad here and offer two different (but not diametrically opposed) kinds of advice: The quick and "easy" one is to decide to let it all wash over you. "What's done is done and there's no good in crying over spilt milk. Make the best of a bad job..." Be polite to all and avoid those who are nasty to you. Step around them, as somebody has already advised.
The other is to go on the offensive: out yourselves to everybody else and hold your heads high. (I think that River's the only other member who's suggested this.) Give your girlfriend a prominent role in the wedding and show that you are all proud of each other. I know that this is the really rad alternative, but consider these advantages:
1) You're stealing your sniper's ammunition.
2) You don't have to WORRY about people (especially your fiancÚ's family and all the grandparents) finding out from the sneak.
3) You're not going to make your in-laws feel that you were being sneaky and dishonest by keeping quiet until after they'd paid for the show. The way things are now, they will find out someday. Wouldn't it be better to be honest and trust to their affection? Otherwise they will resent.
4) You have the chance to inform all your invited guests, let them decide whether they want to celebrate and share your joy on this day, or decide that it's a travesty they can't condone.
5) Those who have already bought plane tickets have the chance to choose between
i) trying for a partial refund (the sooner the better);
ii) flying but going to a theme park in your area (?) so that the money didn't all go to waste; or
iii) demonstrating that they do love you and want to be there for you.
6) Opens up whole new possibilities for the reception. I mean, I really like the Quaker-inspired service idea, but afterwards... hey, let's party! (No I don't mean an orgy, but after a "catastrophe", there's sometimes a euphoric high. Ride it and let your friends ride it.)
7) It lets you send out [rush-job / e-mail] codicils to the original invitations, explaining your continued love, your continued commitment (to each other and to those who choose to remain your friends), and your continued wish to share this special day with them. Add that anybody who plans to attend and be nasty would do better to stay away because
a) you've already received so many messages of loving support that any would-be-abusive guests are going to find themselves mightily outnumbered;
b) you've hired as bouncers the toughest honchos from a specialist firm called "Poly Gorillas"; or
c) use your imagination! Don't let little quibbles about truth and fib stop you.
My gay brother came out to me and to another brother. Then he was outed to our parents by 2 arch-conservative brothers. After that he told me: "You know? In a way it's a relief, because now I can stop worrying every time that I go on a Gay Pride march that they might see me on their television screen..."
Whatever you decide, I agree wholeheartedly with others' comments that this is YOUR day. Whatever you decide, take the reins in your hands, and don't let other people highjack YOUR wedding. Even if your future in-laws withdraw financial backing, YOU decide how to economise without cheapening the occasion. (One idea would be to take up a collection - small as it may be - among the friends who ARE supportive.)
It's quite apparent that there are absolutely no dissenters on here to the proposition that your father is a real arsehole. So I don't really even need to have written that. YES I DID! For my own good. It feels good to have chimed in on that one!
I assume that your mother is either more supportive or at least less condemning than your father, seeing as she hasn't withdrawn the offer of her home as the venue. Why not invite her along to talk to the future in-laws? You know: something like, "Well, I can tell you that it was a shock to me, too! But I'm coming to terms, I'm coming to terms. I'm not going to stop loving them at this stage, and I hope you agree with me on that..."
I was always able to discuss with my conservative-but-caring mother my support for my gay brother's relationship. (I told her that in my opinion it was the healthiest one in the whole family.) But as soon as my dogmatic ("Anybody who disagrees with me is just plain wrong!") father would walk into the room, I'd shut up, knowing it would have been wasted breath... Ironic then to be told that he "had a thing for men in uniform".