I think it might be helpful to distinguish between planned and unplanned pregnancy. Planned has different approaches to unplanned.
To planned, I feel there's some information missing here... Does she want a child? Does she want a child with him? Does she want to be open to the possibility? Does he want a child with her? Does he want to be open to the possibility? How do they feel about you wanting that not to happen, ever? Is this issue concrete or hypothetical?
To unplanned. I'd like to link you to polyamorous misanthrope
, who had a good guest post. The hierarchical terminology may not apply, but otherwise it's relevant to your situation. I think you are smart to think about the possibility of unplanned pregnancy, because once it happens, it will be out of your hands.
You seem pretty clear about the fact that you would not be okay them having a child. Furthermore, you will probably never be okay with it, because for that to happen, you'd need to pursue it. That is, it very likely won't change because you are not okay and do not wish to be okay with it.
So, that leaves you with options. Deciding between them is, I guess, based on how you feel, what you want, and what you value. What is your ultimate goal? That you will be the happiest or for everybody be happy? Supporting your partners' autonomy or have them compromise to accommodate your feelings? Here are the options I can think of.
1) Ask them to not be in a relationship. Unfair, unreasonable, but you can do it if you want to. I consider this ethical as long as you acknowledge the fact that you are being unfair&hypocritical in the request. It's up to them to decide if that is something they are willing to do. Also, if it is, it might lead to resentment on their part, which is a risk you would be taking.
2) Ask them to refrain from PIV sex. Again, up to them to decide.
3) Ask them to do their best to prevent a pregnancy. Decide that if, despite of that, she gets pregnant and wants to keep the child (might want to ask if that's what she would do - will help to evaluate the risk), it will really really suck, but you'll just have to deal with the reality in one way or another (including potentially ending your relationships with them). Keep them in the loop about how you feel about things, so that they can make informed decisions.
4) Decide that both whether they want to be in a relationship and what direction they want that relationship to take are their decisions to make, and you will just have to deal with the reality in one way or another (again, including potentially ending your relationships with them). Keep them posted.
5) something else I haven't thought of?