Actually, the libertarian argument against infanticide is one of the better philosophical arguments out there if you’re not going to come at it from the point of view of religion. Not everyone believes in God. Sad, but it is what it is.
Responsibility and freedom go hand in hand. There is plenty to disagree with when it comes to libertarian philosophy (I’m not a libertarian but am sympathetic to some of their views), but their argument against infanticide is one of the better ones. It’s a shame it’s not better known.
It isn't known because a small number of dissenters are not meaningful to what libertarianism is, I assume that the democrat party has some pro-gun people in it, or even anti-"infanticide" people in it, but they aren't making a difference, just as your anti-"infanticide" people don't make a difference in libertarianism.
It is dishonest to try and portray libertarianism as not being full term pro-abortion.