I think it’s a weird idea that appointing an Obama-supporting Catholic (ergo, not strictly pro-life voting) to be ambassador to the Vatican would be an insult. Henry Farrell argues that so far as Catholic orthodoxy has been defined on the issue, voting for a pro-life Catholic can leave you a Catholic in good standing.
More importantly, our ambassadors often have major disagreements with the countries they are assigned to. It would be quite a poor American ambassador to Syria whose opinions were consistently congenial to the Syrian government. The Vatican is made up of much nicer people, but that just means that there are many more issues where an ambassador’s opinions should naturally align with the Vatican. On issues where they aren’t aligned, the situation should be the same. Under any administration, Roe v. Wade, puts the US position out of line with the Vatican, and an Obama administration makes the disagreement sharper.
The policy of being delicate with the Vatican only makes sense so far as the issue is merely symbolic, and I doubt that’s the case–the Vatican is too political for that to happen. If, for some strange reason, the Vatican would prefer a non-Catholic ambassador to present American pro-choice principles, there’s no reason not to appoint one–surely we should do what will get us the best results. But this means positing a very weird sort of irrationality on the Vatican’s part–any ambassador worth having will be defending policies that the Vatican disagrees with. If they’d prefer to avoid having a pro-life Catholic do that job, it’s hard to see what motivates that preference.