I think, in general, it's not appropriate for a roommate to impose their will on the other roommates with respect to something generally accepted in society, without stating their preference/rule up front at the time of lease (/roommate agreement) signing. If you don't want your housemates drinking alcohol, or having guests of an intimate nature, or you want everyone to keep kosher, or whatever... But, you say that you're fine with it in the general sense, it's just this one-time, unforseen event that is bothersome (but perhaps in the future seek roomies who'll agree to not throw big barbecue/grilling cookouts at the house?)
I think you are doing the right thing in just extricating yourself from the situation for the duration. I would advise also avoiding FB for a few days (if you frequent it) because if most of your friends are going, they will probably post photos (a pig roast is a novelty and big deal to meat-eating participants), and unfriending *everyone* is probably too much hassle. If your roomies are decent people, it should be perfectly fine (and effective) to tell them "Guys, you know I'm ok with you having the event here, but it really makes me physically uncomfortable to think about, and I can't help it, so I'd really appreciate if you just didn't discuss it around me if possible." Then, try to keep to yourself a fair bit leading up to it, to give them the space to do that (vs sitting in shared spaces all evening every evening; I don't know your habits). I also wonder if them referring to it as "the party" would help at all? Then at least if you overheard them talking about drinks, chips, and so forth it wouldn't automatically conjure up that mental image for you?
I hope you find a friend (or few) to hang out with, but if not, could you crash with someone even if they are going, just to have a home base away from the distastefulness, and do your own thing during the day(s)? This is obviously a big inconvenience for you, but IMO the key to good roommate relationships is being as accommodating as you can. I would routinely inconvenience my housemate by having various friends spend the night (no, actually friends, not *friends*) but I'd also watch her dog fairly often and do dishes. And she'd share her homebaked bread and pies. Mutual accommodation all around!