If a girl said that I was shaggable, I'd probably take it as an insult, that she was meaning I was a slut. Likewise if a lad said it, I'd be a bit suspicious, but it totally depends upon the situation, if it was just someone eyeing me up and saying it, I suppose I'd be flattered.
At this age, I don't think many people would describe each other as marriable, I guess if someone said it, it would be nice though.