WHOA!.... Your guild leader is waiting for a reply and you probably want to avoid pissing off the guy right away with a well timed "FU_K YOU" so what to do? You've got two main choices:
#1) Meekly accept, sacrificing your personal happiness for the needs of the guild. After all, isn't the group need more important then your own?
#2) Screw that, you're a Ret Pally and they'd better accept it... better they get to accepting now!
Either of these choices comes with some potential consiquences. Choice #1 will leave you in the guild and you'll start running more PvE Instances with your friends. Of course you'll be the group healer and your dreams of RET will go bye-bye... and the armor you've acquired for ret will be tossed aside for healing gear.... Worst of all, you won't get much advise from my blog on Holy :)
The consiquences for choice #2 will vary depending on your guild, either they'll say fine and not bring it up again, they'll say fine and stop inviting you to as many guild functions, they'll ask you to leave (or make it uncomfortable enough that you'll leave).
Of course you'll have to decide what's most important to you. But I will give you my opinion on it.
Tell your guild you will NOT respec. Don't bend over for some guy's request! You haven't asked anyone in your guild to respec for you, and you expect the same courtisy from other people! If they need a healer so bad they should get out and recruit one.
Take a sec to think about why they're asking you in the first place.. probably to run instances. If that's for you then great. But if you've embraced PvP then you don't want to waste 5 hours in a big Kara run anyway. You probably also don't want people setting your WOW schedule for you.
If they threaten to toss you out then go ahead and type in "/gquit" and get out of there. I'll let you in on a little secret... There are plenty of guilds out there oriented to PvP. They'll gladly accept a Ret Pally and will help you run BG's get Arena Teams together and will probably be more condusive to your play style.
It's easier to say "go ahead and quit" then actualy do it, especially if you've been with the guild for a long time. Kind of like moving to a new neighborhood, you won't see your old friends any more.... But you'll make new ones, maybe even better ones that understand you and your spec.