Lyric discussion by Atomicwoof 

Cover art for Siren Song lyrics by Bat for Lashes

This is my interpretation:

I think the song speaks of a girl who is very loving and nurturing(as the song describes), but has constant doubts and fears for her relationship. I think the siren stands as a metaphor for these doubts and fears(sirens are known to drive men insane as explained in a previous comment). I believe her greatest fear is that her boyfriend will leave her because she thinks she's not good enough(she's got so much wickedness and sin) for him and she constantly asks confirmation of him by saying things like: "Tell every last boy that you're my man". She tries to soothe herself by saying things like "I think I could be all that you want" and "I'll try not to let you down" (The "I'll try" and the "I think" ofcourse stand for her insecurity). She's very aware of her insecurity, and that might just be her greatest fear. She thinks her insecurity might be the thing that drives him away from her ("And the stars are exploding in your eyes"/"It won't be long until you break" -> The last drop for him).

I interpreted the line, "tell every last boy that you're my man," as a sign of fidelity, not insecurity...