Thanks for sharing...
your position is a tough one. Here you have this ideal partner in your life, and after having a rough previous relationship, are very happy to have found him. He wants something that you wouldn't choose on your own, but given that you're trying to be a 'good' partner to him and want to make him happy, you're considering something you've never considered before. it falls way outside of your normal scope, and even though you sound like an open-minded person on the whole, you're a straight woman who is being asked to consider something that is not comfortable/natural for you.
it makes a lot of sense that you're coming up against a lot of concerns, and that you're wishing to draw boundaries and "call the shots" to a certain extent. In all practical ways, you'd be giving up a lot of power in the relationship, and putting yourself in an unknown and potentially very unstable place, and you'd like to hold on to some form of power/choice in this situation. you have a right to these feelings; they are real, and they are legitimate.
My first observation is that (in my opinion) it is too soon to be introducing another person into your fledgling relationship. the theory is that until you've been with someone for around 18 months, you don't even really know them/your long term relationship potential. is there a reason that you two aren't getting the opportunity to just be together as a twosome for a solid amount of time before opening up your relationship?
I hear you saying that even though you're pretty damn uncomfortable with the idea, that you'd be MORE willing to go along with it if you could choose the woman that you'd be opening up your relationship with. I hear this in conjunction with you saying that you identify as straight, and have never been with a woman before. I see some challenges with the second part of this, but understand your desire for the first part.
i think the thing that you need to consider is that no matter who "picks the woman" you'll still be a) opening up the relationships (which you're not comfortable with b) have a woman that you're seeking out under the pretense of a triad (you've said that you don't want a vee). this means that you'll be having sex with a WOMAN that you don't even really want in your relationship period. this is very problematic for you, for the other woman, for your partner, and for all engaged relationships (yours/his being the primary concern at present, as it's the only established one right now).
i think the thing to concentrate on is that people/love/relationships evolve in ways that you might not anticipate. even if you pick the woman, there is no guarantee that it won't end up in a vee anyhow (ie - they fall deeply in love, you find her annoying, and end up watching them be in a relationship that you're not really involved in despite your best intentions).
listen to your own heart. If poly isn't something that you want, you're getting into some dangerous waters by pursuing it. it's not fair to you, or the potential gf to be putting yourself in a place that you're not open/comfortable with. perhaps your energy would be better spent considering all kinds of options for poly that don't involve a triad at all.
my latest foray into poly was the poly-fi triad. i AM into women, and DID choose the woman, and still ended up in a messy vee that drove me crazy and put some intense strain on my five year relationship. poly-fi triads are fuckin' intense, and very few survive long term - it's a nice idea, but in reality, they're pretty hard to successfully sustain.
Options! Maybe you should get another boyfriend. maybe he should have his own gf. mebbe you two should be in a quad with another couple. maybe swinging. maybe put up a 'bi curious' ad on okcupid/CL and see if you could even go for a woman with him. maybe attend a sex club. There's lots of ways to explore sexuality and try poly without going for a poly-fi triad.... it's like trying out for the major leagues without even ever having held a bat before.
Don't push yourself too hard. there are a lot of amazing men on this planet, and if this not the guy, it'll be fuckin' awful to let him go, but you'll move forward knowing one more thing that you don't want to deal with in the future. Just know that you have a shit-ton of options, and pushing past your comfort doesn't have to be option #1.