At what point are we pretending fetuses aren't human? At some point they become human, but far sooner than that are they "routinely" aborted, when they're nothing more than cells. Anything after that is an active refute of broadly accepted science.
The desensitization comes from people having so much access to information all the time and often without filter, to the point where abolishing basic human rights becomes a normalcy. Without going off topic, the more one sees atrocities of any form, of any severity, the more they become immune to it. Yes, the same could be said about being pro abortion, but in light of stripping away the concepts that make already established people human? I'd much rather extinguish life before it becomes sentient than take away any sense of humanization an already established sentience already has.