I am a female in a V with two males at the moment. They both identify as heteroflexible (e.g. they are open to the idea of sex play with other males but not particularly drawn to it) and they are not involved with each other at all. They are just good friends. My fiance and I have been together for four years and my boyfriend and I have been involved for about a year now and officially dating as boyfriend/girlfriend for about nine months. They are both open to explore other relationships, and at the moment my fiance is exploring a romantic connection with a married woman (her husband is fully aware and okay with how things are going so far).
Other long term relationships don't necessarily need to threaten your standing with her. NRE can be a scary time, and it might do you some good to do some reading around the forums about it. Both of you could make the mistake of thinking the intensity of NRE means she is more in love with her new partner or that she is falling out of love with you. It never lasts though. NRE is impossible to maintain for any more than about two years for most people, and can be as short as 4-6 months. So, in a sense, a relationship that lasts long enough that she falls out of NRE could end up being less threatening to your relationship. However, this is a very difficult limit to put on someone else's relationship. If you have a short term limit that is more than just a one night or one week deal, then you run the chance of them falling in love or getting caught in NRE. At that point, it is difficult and painful to just cut someone out if the only reason is because it is against the rules to date them any longer than that. If it is the case that you don't want anything serious on her end, then it might be better to put rules in place that make the relationship more like swinging, where physical play is allowed, but you take steps to make sure you don't explore other emotional connections.
And just like Jane said, even if it was a woman that your partner ends up in a relationship with, you might not be attracted to that woman or that woman may not be attracted to you. So you have a chance of being in the same situation of being in a V whether your wife's new partner is male or female.
I would talk to your partner about your needs here. If you do not want to be in a mono/poly relationship long term (where she is poly and you are mono) then you need to express this to her. Perhaps you can have that arrangement for a few months, and then she can tell you ways that you can show her that you are trustworthy now in order to build up to being able to have other experiences outside of your relationship for both of you and not just her. If this is something she outright puts her foot down about and it is something that you don't think you can live with and still have your needs met, then tell her you do not wish to open up your relationship at all and then it will be her decision to stay in a closed relationship with you or to break up to seek a poly-style relationship.
I guess from my perspective, I am never going to leave either of the men in my life because I found something "better." I have good relationships with each of them, and I would not end those relationships unless something was wrong in them. If we have problems, I work on them and try to fix them and find a compromise we can all live with. I don't just skip around to something different. I've overcome some really big hurdles between my fiance and me and between my boyfriend and me. This doesn't necessarily mean your partner isn't going to cut and run if/when the going gets tough, but if she is that way anyhow, then I don't know that you have any more security in monogamy/monoamory then you would in polyamory. If she doesn't want to be with you, she is going to leave anyhow.
It may be a good idea to see a poly-friendly relationship counselor and talk about some of these past infidelities and trust issues and make sure that the two of you are in a secure space before you would decide to open up your relationship. Best of luck to the both of you. :-)