To provide some additional context, Girl 1 and Boy 1 are thought to be dating by lots of people (they are very close to one another, but they are not officially dating at least). 烈 wanted to make Girl 1 his girlfriend so he challenged Boy 1 to a fight, Boy 1 cheated and won. They were then discussing this later where the following occurs:
Girl 1 「反論されても……烈さんの男気に比べるとどうしてもなぁ……」
Girl 2 「んん！？ まさか浮気の予感！？」
Boy 1 「面白くない！！」
I think I understand the usage of それはそれで, however I am not quite sure what she is saying is 面白い from the previous utterance 「まだ付き合ってるとは言ってないよ」.