Absolutely. As soon as one person starts trying to control the narrative, it all comes tumbling down. It’s much better to have boundaries than rules, so if you you both have a boundary where you might not be comfortable with a third person touching your partner, a threesome sort of becomes a non-starter really. However, it’s not a boundary for her to say “I’m not okay with other people touching my fiancé”: boundaries are limits agreed within the relationship to maintain respect towards your relationship, rules are about controlling your partner/other people to eliminate your own feelings of jealousy, and are inherently disrespectful to both partners and other people. It’s much better to set boundaries and tackle that jealousy head on, through solid communication, boundaries and confidence building.
So, going back to my example with the last couple, I should have said to the guy “I’m really sorry, I don’t get on the relationship escalator with non-residing partners. I may be your partner, but I am not your girlfriend. Please don’t refer to me that way” - that’s setting the boundary that would have made my husband, my primary partner, feel more secure. I failed, and the relationship failed because of the jealousy and confusion that followed over exactly where our boundaries were. They kept a huge secret until after we got together anyway (they were trying for a baby at the same time as opening up) and so it’s not only my head on the chopping block, but that’s sort of by the by. My point is, boundaries work much better than rules - they treat your partner with the respect that they deserve.
So, if you would want to be involved with the other girl too, there has to be an agreement as to how far this can all go, that you’re both comfortable with (maybe making out is okay, but penetrative sex is not?). Your fiancée needs to find where might be okay for her, and what might be too far, if indeed she can. Maybe she wants to explore with another woman without you, so can you also explore with another woman without her, if you wanted to? It’s one thing you not wanting to because you love her, but if one partner is controlling the other and their extra relationships, it’s no longer ethical and is likely to breed resentment somewhere eventually. That could then lead to outcomes far worse for her than you making out with another girl in front of her. Not speaking from experience there, or anything 
remember there’s no rush for these things to happen so take as much time as possible to be fully decided on it.