Lyric discussion by LacyLand 

Cover art for Only You lyrics by Ellie Goulding

Very simple.

He's the only person that can make her feel helpless, in pain, the "only animal I couldn't fight" because she is so in love with him, he has complete control over her.

"I'm on my knees" is in no way dirty, it's him hurting her and having so much effect on her that she's forced to fall to her knees from the impact of him.

My Interpretation