"St. Paul never proposed that two believers got to divorce and remarry."

Yes, but he never proposed that a baptized person get to divorce and remarry either, if his first spouse is an unbeliever.  Yet that is precisely what the Petrine privilege allows for, first introduced 1600 years after the words of Jesus and Paul!

Most translations of Matthew 5:32 don't say "unless the marriage is unlawful." The Douay-Rheims, following Jerome's Vulgate, has "excepting for the cause of fornication."  

So the traditional catholic position is:

1. Divorce and remarriage is ALWAYS adultery. No exceptions.
2. Matthew seems to allow for an exception in the case of adultery, but that's based on an incorrect translation that persisted for centuries even in standard catholic bibles.
3. We do allow for the Pauline privilege. OK one exception, but it's in the bible.
4. We also allow the Petrine privilege. OK, two exceptions now, but hey that's an uncommon one.

Not so simple anymore.