I think the underlying cause is that wherever you have a group of people interacting with each other (even when this is only two people), you have a situation where each participant in the interaction wants different things. The outcomes that you as an individual want are 'right', and those you don't want are 'wrong'. Your own 'right' and 'wrong' will differ from other people's 'right' and 'wrong'.

In this situation, for Alice, the idea of Frank being with Marry is 'wrong'. It doesn't really matter what in her mind the 'right' outcome is, whether she would like to be with Peter but also stay with Frank at the same time, or whether she doesn't want anyone to be with Frank, or whether she just doesn't want her friend to be with him. It's enough for Alice that Frank being with Marry is 'wrong'. She has not reached this definition of 'wrong' through any logical reasoning, it's simply an emotional position. What she then does is to try to seek any reason she can find that might support this conclusion. She then decides that Marry's action is not compatible with the rules of friendship... because this is a reason that supports her desired conclusion, that Marry+Frank=wrong. Alice's reasoning is superficial and inauthentic. Her 'reason' for Marry's action being bad is not a reason at all, it's simply a mental prop to support a pre-established conclusion. It happens all the time. Consider for example the painful logical contortions that creationists will go through in order to deny the truth of evolution through natural selection.