Lyric discussion by tambourine(wo)man 

Cover art for Mayfly lyrics by Belle & Sebastian

The whole use of the metaphor of a mayfly makes me think the girl that the narrotor is "privy to" is not really what he looks for in a long-term girlfriend. Mayflies only live for about a day I think and so it's as if he'll soon get bored of her, but he takes advantage of her infatuation with him when his girlfriend is not around. That's what I reckon anyways...