Um... I have a Love problem.

You shouldn't be a jerk, you sound as if you've already decided for your GF over your friend, so don't give this girl ( the shy one) false hopes. If you want to leave the door open, just act like you've acted until now, don't get weird.

Stick with your girlfiend!!! cheating is not an option!

call a meeting and get everyone on the same page, guessing that your GF doesn't know about this. get them all in the same room and with snacks, sugury snack soften the blow. tell the GF that your friend came onto you, don't lie, lie for the little things. tell your friend that you like her in a friend and not as a lover. best case scenario,they are ok sharing you and you get the mythical harem ending. worst case scenario you lose both and they try to kill you in your sleep.

one more thing, wear a cup just in case.

do I hear a `I wish it would have been the opposite way`?