In a mixed marriage, there is 3 possible outcomes: 1. Each party practices their own religion 2. One party leaves the catholic church because of the other party. 3. One party brings another party to the catholic church.
It isn’t wrong to have mixed marriages, but it certainly isn’t easy. I have met many catholics who are more open to mixed marriages than protestants, but the ultimate outcome is for number three. The priest might have a bias as he doesn’t want the scenario of 1/2 to occur. There’s also issues of social justice, in the case that catholics marry non-catholics, then catholics who are searching for catholics might inevitably struggle to find catholics, especially for a diocese that is small.
As for non-catholic christians, I think might be bias again. Some examples are going to church on sunday for non-catholics is quite of an option, many doctrinal issues such as veneration of mother mary. Perhaps, he has a negative view against protestants, because he might have bad experiences with them in the past.