The "rule" is-
You, she and he need to discuss what it is EACH of you wants/needs to be happy in a relationship and figure out if those things are compatible.
That's it. IF it is compatible-great move forward. If it's not-then be "just friends" with whoever it won't work with.
Different people need/want different things. The key is finding people who are mature enough to REALLY know what they need/want and then being sure the dynamic of the relationship allows for and promotes discussion of those things.
"Love As Thou Wilt"