<human>: Can you condense the text into a few key points?

A little background, my girlfriend [f24] and I [m22] have been together for about 3 years. She recently moved in with a good friend of hers into an apartment together. I spend quite a bit of time there bc of my SO, and the more I interact with her friend the more sexually attracted I am to her. There have been several occasions where we have gotten drunk together and they have made out and one occasion where they were both sober. This I feel has somewhat sexualized my feelings towards her as a friend. I've caught myself thinking "fantasy" type things involving her and I almost always feel guilty because I know if my SO were to hear those thoughts it would kill her. While it isn't an obsession (probably once or twice a month I catch myself) I still like the idea regardless of my guilt. 

As for how I feel about my SO, I love her more than life itself. We are planning on getting married as soon as we are both finished with school and can move somewhere else. I have no doubt that she is who I want to take care of and grow old with. My issue is that while we are extremely open and honest about our thoughts and feelings so I've said something to the effect of I find her attractive. She said she understood that I'ts not something I can help (noticing the looks of another girl) but I could tell it hurt her self esteem. 

I just need someone else's thoughts on how I should handle this. Do I admit I've had sexual thoughts about her friend? How would I approach that? Or is it just one of those things you keep to yourself to spare their feelings?  

And just to make sure I'm absolutely clear, it is purely physical to me. I'm not trying to start something or initiate in any way. I value our friendship and would hate to see that dissolve in any way as well.  I love my SO and would never ever do anything to hurt her, I just don't know how to handle these kinds of thoughts and feeling specifically.
<bot>: I'm physically attracted to my SO's friend and while I've admitted it before in honesty to my SO, it really affects her self esteem. Just need some idea on how to move on.