Thankfully his is a priest. In charge of hearing confessions, not telling us what we should confess. A vote for McCain is a vote to destroy the pro-life plank (as well as many other conservative ideals) in the GOP platform. I think his eventual nomination is already the death knell of the Republican party. Who needs another liberal, fiscally irresponsible party. The Democrats have that base cemented in (see Obama's reaffirmation of the party only unchanging plank.)
Me? I won't call any vote a sin. I don't know anyone's heart or conscience. But I know I can't vote for anyone that supports abortion. That narrows the field considerably. I am not confident of McCain's stance on that issue, but he claims to be pro-life. So I look at the other issues. And I won't vote for him based on those issues. Pro-nation building. Pro-big government. Anti-freedom. Pro-amnesty. That is enough for me to still vote Ron Paul. He may not win, but the GOP can look at all the votes they "could" have had.