I'm not really sure how to translate 「ぐちゅぐちゅがとまんなくなっちゃって……」. Anyone have any ideas?

Maybe something that reads better, like "I can't stop fingering myself"? That wouldn't stray too far from technical accuracy while working with English's lack of onomatopoeia (I assume that's what ぐちゅぐちゅ is the noise for...).

She's wearing some pretty extravagant underwear to be unaware of masturbation- then again, who can tell with eromanga? Maybe "I can't stop rubbing here", since "this" sounds like she's talking about a separate object.

windward said:
She's wearing some pretty extravagant underwear to be unaware of masturbation- then again, who can tell with eromanga?

There actually is at least one like that. "Onegai Sister", by Saegusa Kohaku. The little sister doesn't know what masturbation is, and gets freaked that she's wet downstairs, so she goes to get help from her brother. Brother gives in to his urges, and it turns out that while she doesn't know what masturbation is, she does know what sex is.
Ero gold.