Fail Date: Charl - Just out of spite. I'm fed up with all the IS fanboyism here, it's really not worth it.
Sex Once: Mio - I've never seen an episode of K-on, so I only have doujins to go with when imagining her personality, and we all know those are generally unreliable, so I'm taking the safe choice.
Marry: Ibuki - because she's the only one left. One of my least favourite heroines in Hoshizora, but all Hoshizora heroines are cute and quite acceptable personalities, so it's not a problem.