I am all for telling the people I love that I love them.
Femmes are not fragile. BTW. Or more emotional lol.
Some of us are, but just as many of us are not. Lot's of butches and lot's of people in general are emotional.
But back to the subject, if she knows she is in a poly relationship with you and everything is in the open and you love her, why qualify it? Love adds to love, it does not have to be I am INNNNNN love with one person and just love this other person like love has degrees.
If she knows the situation and you have been honest, it seems a bit patronizing to try to protect her from herself.
I would be more likely to say I love you and here is why and how.