Like so many things involving beards, give it some time. Can you convince her to give you a week to "consider" how you want to trim?

Over that time your mustache and beard might grow on her, or lay down, or curl nicely, or whatever. Or, she might be distracted by something shiny...I know I can be distracted easily.

Barring that, you have 3 choices.

1) Razor blade. Please do not do this, because your beard shows a lot of growth, density, and promise.
2) Clipper. Whether a beard trimmer or a hair trimmer, the idea is to trim it into shape.
3) Shears. Also known as scissors. You can trim very judiciously using this tool, and achieve a very different "look" than the absolute uniformity of clippers.

Do you know how to blend using a clipper-over-comb technique? Rarely does hair, whether on the head or the face, look best when trimmed entirely to a uniform length.
 
If I was going to trim anything, it would be to blend your sideburns into your beard, seamlessly, so there is more of a relationship between the two.
 
P.S.  Is that her in the background?  She's cute.  You're a lucky man.