Bob gives his sister Janet money to buy food, knowing that she'll probably use it for drugs instead. She does use it for drugs, gets high and gets pregnant. She has no idea who the father is. Well, we don't want the kid to suffer, so let's make Bob help pay for the kid because he helped her get high, which led to the kid.
Yup, makes perfect sense. I mean, now that we've decided that the kid is the most important thing to the point of disregarding all other logic and reason.
Would you like some more straw? The straw man you're trying to build seems sort of floppy.
Do you really have problems with the idea that sex that results in a pregnancy is responsible for children in a way that other actions are not?