I'm not sure, I'm mostly just shooting in the dark here.
Some people I see say "the ship is paedophilia because Spinel's thousands of years old" do so with the mind that Spinel (along with all gems we've seen)'s a fully mature woman which is a reasonable take away, so far we don't see childish gems except when Amethyst first popped out, but a gem being childish/adultish when they first form isn't really a concept beyond that.
While some who ship it took Spinel as being "teenage coded" (around Steven's age or just a bit above a la "this character that looks 8 years old is actually 1200 years old") since they shared a lot of the same struggles and were able to connect on a somewhat equal level and despite being thousands of years old timewise (which means almost nothing to gems in a lot of cases), Spinel was pretty stagnate due to being in one state and waiting.
So what stops some people from just being fluid in their interpretation is that after someone in the first group imagines Spinel in an adult situation, putting that on par with someone close to Steven's "pure" character grosses them out, making them hate the ship even more, instead of just being like "alright, different strokes for different folks".
Cause someone could ship them in the "purest" way possible (aged up, consensual, no messy break ups with Connie, no interaction with Steven pre-puberty), but someone from that first group would still find it gross.
This doesn't take into account people who don't give a shit about putting these characters in adult situations at any point, or people who hate ships purely on a "it's not for me, so it shouldn't be for anyone" level, though. Or people who plain don't like steven/gem ships because the literal age difference is a bigger deal for them than just a matter of interpretation.
It's just funny to see someone go "I want Spinel to be my GF" and then get really mad at the slightest hint of gem/steven shipping, like Lapis, Peridot or Spinel.