One example I'd propose of Catholics letting sentiment pass as argument is the characterizing of opposition to abortion as being "all about babies."
The problem I have with this is twofold: It's not true; and people know it's not true.
It's not true in the very narrow sense that, properly speaking, embryonic humans aren't "babies." In the abortion debate, we call them babies for rhetorical purposes; in common speech, we call them babies because a happily pregnant woman will stab you with a fork if you ask her about her "fetus." But in its primary meaning, "baby" denotes a born child, and it is sentiment, not physiology or philosophy, that calls a 14-day-old embryo a "baby," just as it is sentiment that calls a two-year-old child a "baby."
Moreover, it's not true that opposition to abortion is "all about babies" in the broader and more important sense that it's about children, and mothers, and fathers, and other members of a society. To focus on "babies" is to ignore, not only that we don't start out as babies, but that we don't remain babies. It's to ignore the role of parents, not just in pregnancy, but in raising children. It's to ignore the role of society in supporting the parents in giving birth to and raising their children.
And, as I said, people know this. People know that they do not feel the same way about the idea of a 14-day-old embryo as they do about, say, a three-month-old baby (by which term everyone understands a baby that was born three months earlier, not one that was conceived three months earlier). If the argument is, "Well, you should! They're both helpless babies!," the counter-argument will be, "Tough."
People also know that, "But it's a baby!" is no answer to a woman who does not feel capable of giving birth to and raising a child. You can't just tell someone to swap out their feelings for yours.
Last week, there was a Twitter campaign to normalize abortion by having women tweet using the "#ihadanabortion" tag. In response, there was a pro-life campaign to tweet using the "iamprolife" tag. Some of the latter tweets include:
#Iamprolife because babies cant say mommy please dont kill me, they need big people to say it for them.
#Iamprolife because babies make everything better, marriage, family, hugs, laundry(cute little clothes), first steps, toothless grinning....
#Iamprolife because babies are super cute!
#IAmProLife because I love babies. All babies. Even the "imperfect" ones.
As expressions of sentiment, I can't object to them. If feelings like these are what fuels people to good works, super!
But we should recognize that these sorts of statements aren't arguments against abortion. Some of them infantilize the very real problems many pregnant women face. ("Think of the cute little clothes you'll need but can't afford!") Some of them reinforce the prejudice that pro-life people don't care what happens once a baby is born. (Do we love all teenagers, even the "imperfect" ones?) Some of them are non sequiturs. (You want super cute babies? We got super cute babies.)
Granted, the above tweets were not offered as arguments for opposing abortion.
Granted, some people can be persuaded to oppose abortion by appeals to their feelings about babies.
That said, feelings about babies do not constitute a valid argument against abortion, and those who argue against abortion should know this and be prepared to offer valid arguments.