That rule being that I cannot fall in love with him.
Is this swinging or polyamory? Friend with benefits? What?
We can't help falling in love. It's a brain thing -- the cascade of hormones and pleasure goody feel goods. Google that, understand that. The neuroscience of falling in love. Because they (your poly peeps) don't seem to get that if they are making that a "rule" that you cannot fall in love.
And you -- remember that just because you fall in love, this doesn't have to be the only love in your life or even that you have to ACT on it!
If you stay in rship -- brain cascade will get you through the first year on crushing / love drunk. It's fun to feel.
It's not enough to build a life on, but you don't sound like you want to build a life with this man. Sometimes it's enough for a while -- people come in your life for a reason, a season or for life. And the lifers (A season of a lifetime) are rare!
You don't sound like you have laid out a framewor
k for how to be together in right relationship for X amount a time.
Spend time figuring out YOU wants, needs, limits for the next year(?) of this relationship. You can always review in a year and see what tweaks are needed for the next chunk of time. Then present them and check in with their wants, needs, limits. Even if you are not dating her you are in a metamour relationship with her and she may have wants, needs, limits to share. At the very least about the calendar scheduling and safer sex practices that could affect her!
Make SURE that the fiancee knows about you. You do not have to be friends, but it's nice to know all parties know it's open rship and you aren't being used like a cheater-y thing, right? Use safer sex practices. And do not promise exclusive. Continue to date. Take care of your emergency preparedness YOURSELF -- STD, unplanned pregnancy, etc. You do not need to have some kind of oops and be left in the lurch. I hope you are not. But let's keep it real. You are a dating single woman, and you take care of your own stuff. You are responsible for emergency preparedness.
You are young, and this IS your best dating time -- while a student in college with more time than most adults. Enjoy your young adult dating adventures. But see if you have a support group on campus for lesbigaytranspoly folk to help you navigate alternative loving. Explore resources like http://www.morethantwo.com/