Ambiance,
I'd probably respond with something along the lines of, "Hey Jane, I'm actually trying the whole 'monogamy' thing with a girl right now... I'm not sure if it's really my thing, but I figure I should give it an honest go for awhile and see how things work out. Keep my number saved though in case you hear from me in the near future

"
There's probably different ways to go about this, but it also depends on what your goals with it are -- I'm assuming you asked because you would like to stay faithful to your monogamous relationship for now but have the option to go back to your FBs later if things aren't working out. This text I crafted above was made with that in mind.
Obviously, feel free to delete the text message (and her response) afterward if it makes you feel better to keep it off of your phone. At the very least, the girl will know your situation and be more likely to respond in the future if you hit her up again.
Cheers,
Franco