I’ve thought this, too. Hearing “if it’s not mine, I don’t want to raise it” or some variation thereof gives me the willies.
tbf it is a bit more complicated than that. on its base, the older a kid is, the less likely you're "dad", more likely "steve", and that's also connected to disciplining or advising or trying to be a parental figure. "you're not my dad" x1000. the cutoff point is like at edgy teen phase where you could theoretically have a step dad live out like a old wholesome comedy movie and forge a bond with them, but past that you're essentially locked into steve. that also spreads subconsciously as you being a replacement and not special, which you basically are anyway in that situation, except instead of something like being another best friend when one passed on it feels akin to a coworker you get on well enough with. obviously there's countless situations and what-abouts, like say, a woman with a abusive husband and kid that meets a romance novel protag, or a nearly broken man that meets an angel, but at it's core it's always something where the occam's razor is "... eh nah i'm good". there's also tons of psychological things that add on, like how every relationship deteriorates in value mentally. as in, your school crush that you married and had a awful divorce with after 15 years was very passionate, each one after that is that much closer to a numb state of "they didn't take the garbage out when i asked them/they didn't want to have sex as much or little as i did/they worked too much/too little so i broke up with them". aka you're going to fight like hell for first love, but dale from state farm that was nice you're inclined to not so much.
also, generally in terms of alternative options and from the pov of a male with jealousy/abandonment issues, adoption is the least ick, especially with any research into artificial insemination etc. looking through a book at the most eligible candidate to ensure health of child, also known as what vial of sperm came from the most studly person is way too damn close to feeling like cucking, and if the man gives the woman the same respect, surrogacy feels like cucking your wife. "would you have his children?" is ick. like asking a dude if he'd impregnate your wife. of course it's more detached than that (usually, unless you do the ""natural"" versions i have heard of where your husband is allowed to bang or similar the surrogate until impreg or cum in a jar for use or vice versa) but it's a very emotionally sensitive time for both partners on the end. if the male is infertile, sure "not studly enough grrr" is one mental thing, but more importantly in the context of any relationship, it's "oh god, are they going to find someone else who is? would they tell me if they even did or say it was an act of god we're having a kid after 5 years of trying?" or "oh god, the second date they said they wanted lots of kids, will they break up with me if i tell them i can't have kids?" which can also cause self fulfilling prophecies of just embracing Fuck The World, It Hates Me Anyway, and of course that's not just the man, women have same issues. both partners will be much more likely to be very volatile about "just friends" etc, so it's a very good idea for both to be very reassuring in words and actions.
all that said imo it being "not your kid" is the top of the glacier covering much bigger issues like not wanting to be/feel like the evil stepmom or just steve. excusing pop culture references, no dad by majority is going to have an issue being a joel to ellie from tlou, it just becomes a thing when there's some form of something like having an altar to her real dad that her and her mom talk to for an hour each day while you sit in the garage, or better yet since there's still situations where that's understandable to a point, something like being ""close friends"" with their ex which 99.9% of the time proven by recurring cases just means they are or are going to fuck their ex for whatever reason, for example ""closure"" or labeling themselves unsatisfied
