I think it's less about the fact that Eclipsa went off with a monster but that she, one, left her husband for the monster and, two, is a Butterfly. It' not uncommon for one family to dislike something the rest of the country is mostly okay with.
Even so, Star even points out all of this type of hypocrisy so it's not like the show is glossing over it.
I think the general idea is kinda like this: Most mewmans think of monsters as something along the lines of an unintelligent beast (like our world's stories of monsters often are) and some monsters are okay because they're "civilized" and "like mewmans" and thus don't count as monsters.
I think a good example of this thinking is Queen Moon's reaction to Buff frog's home. She ultimately understands he's "one of the good ones" but he doesn't live in a "real" house. >>97226380
Not to mention Star went from confessing she likes Marco at the party before the battle for mewni, to obsessing over his hoodie, to being annoyed he showed up when she got back together with Tom. She probably had JUST gotten the fuck over him when she realized that, duh, he wasn't the only guy in existence when suddenly he shows up and she's suddenly pissed and confused. Like it makes sense but it's also like "The fuck, Star, at least express your confusion a little."
This show gives me the biggest headache when it comes to the relationships.