She can't be with anyone else
What she hears: "We haven't worked out our insecurities yet, so we're going to control you instead."
If you're coming from monogamy and you don't have any poly experience, the idea of a partner who dates other people sounds scary. What about diseases? Does that mean she will date just anybody? What about commitment? How can you have a family that way?
Polyamory isn't like "monogamy plus more," and if you think about it that way, you'll get into trouble. Sexual exclusivity is something you're probably familiar with and it's probably how you define commitment, but in poly relationships, commitment and exclusivity aren't the same thing. If that sounds threatening to you, maybe the time to get comfortable with it is before you start new relationships.
Think about why people call themselves "polyamorous." They are poly because they reject restrictive sexuality--the idea that if you love this person over here, you're not allowed to love these other people over there. But that's exactly what you're offering. If exclusivity is that important to you, you might have more luck looking for partners who are monogamous, not looking for poly people to be with you.
And disease? Get educated about disclosure, testing, and risk management. You do more risky things than have a lover who has another lover all the time. Driving a car, for example.
She can only have sex with one of us if both of us are there
What she hears: "We haven't worked out our insecurities yet, so we're going to control you instead. Plus, we're super creepy."
Not every polyamorous person is an exhibitionist. Not everyone likes being watched. Not everyone likes group sex. When you tell a prospective third she has to have sex with both of you at the same time, you probably think that's a way to keep one of you from being jealous or left out.
There are a lot of problems. The first is it doesn't work. Even in the middle of a threesome, it's still possible to get jealous--if you feel the other person is digging on your partner more than you, for instance.
It also shows that you're not thinking about her needs. You're thinking of her like a sex doll, not a real human being with real feelings. Real human relationships need a certain amount of one-on-one time to grow and develop, even if you do like group sex.
Trying to control jealousy by controlling how and when sex happens is super creepy. You're saying your needs matter; hers don't. By this point, anyone who hasn't already run for the hills is making tracks in that direction.
What do you do?
Think about how you find local unicorns. Did you go on a first date and then, in the middle of dinner, slap a contract down on the table specifying how, when, where, and under what circumstances your partner would be allowed to be with you, have sex with you, and what the relationship would look like? No. And if someone did that to you, you'd probably leave.
So don't do that to other people. Don't try to script the relationship. Don't try to short-circuit fear or jealousy by saying that someone has to date both of you and perform sexually for both of you. Be open to letting relationships take hair natural course--even if it means that someone is only into one of you.
Yep, it's scary. It can feel so scary that the idea of a closed triad with a bisexual woman can seem like a much more comfortable way to ease into polyamory.
But you can't ease in. Polyamory is going to change your relationship. If you do it right, if you're respectful and flexible and compassionate, it will make your relationship and your life awesome. But it means going outside your comfort zone.
You can't ease into skydiving by slowly crawling out onto the wing of the airplane, maybe inching your way back and hanging on to the tail for a while, until you get comfortable with trusting your parachute. If you don't trust your parachute, don't get in the plane. If you're insecure, nervous, or threatened and you're not sure how to protect your current relationship, build trust with your current partner before you put someone else's heart on the line. And always, always treat other people's hearts well. Even if they don't follow your script.