It seems fairly common for two partners in a monogamous relationship to want different things, ie. either sexual only or a loving relationship, when opening up to polyamory. I would say that being aware of this from the start is a good thing. We have seen so many stories of couples who say they want the same things and then finding out differently when more people are involved feels like a slap in the face. So, good for you. Breathe.
As to feeling like things will spin out of control too quickly for you, well, here is where negotiations begin. It doesn't have to go like, "Honey, this is what I want, so deal with it," and then suddenly she's in bed with someone else. It would be more loving, respectful, and easier to handle if it goes like, "Honey, this is what I want." "Okay, I'm open to it, but let's talk about how that will work for us." "Okay, what do you need to feel comfortable?" And so on.
If you feel like you can consent to being in a polyamorous marriage, then ask her to sit down with you and hammer out what you each would like to get out of it, what boundaries need to be respected, and how slowly you would like to move. Ask her for her patience in making this huge change. You could also invite her to come here and have an account to get feedback from this community as well as voice her needs, wants, and concerns.
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan