I think it is good to look at these texts with a proverbial grain of salt and in the historical context, including but not limited to a common trend of denigrating certain enemies of the Imams (A) on the grounds that they were illegitimate, as well as a heavily tribal culture where once's ancestry generally affected one's social identity and life opportunities. In the time of the later Imams, there seems to have been a particularly strong focus on one's paternal ancestry.

Infants are obviously not born with a conscious hatred of Imam Ali (A) since they have to learn first who he is, and their opinions of him are generally shaped according to what they are told. Of course, one could argue that on an existential level, we encounter Imam Ali (A) prior to birth and thereby develop some relationship. One could also argue that one is born with an inherent disposition towards or against what he stood for (justice, truth, etc). Still, one finds, in practice, many people who are not born within the context of a marriage who do have a strong love for Imam Ali, and many people historically who were born within the context of a marriage who fought him. So it is worth considering these statements in more of a historical/cultural context rather than as absolutes.

It is more difficult to comment in practice regarding the latter since most people don't know whether or not they were conceived during that time. However one can glean the general idea that what happens during conception/pregnancy can leave an imprint on the child, and so one who is siring/carrying a child should be thoughtful of that.