She's marking, what she wants, as already taken. I know why she did it (Keeping the father for her children.) and have a good idea what's another reason (She also likes him.), but this was quite a bitch move. And a stupid one! She showed quite an egoistic behaviour. That might score some negative points. Especially, if you think about the MC position. With that responsibility he lost a lot freedom. Cornering him into a shotgun wedding or going around and telling people they are married even though they aren't even a couple, might send many dudes running. It creates a misunderstanding, that's waaay too easily fixed for the purpose of driving him and his childhood friends apart.
Not cool. "He is the father, but it's complicated.", would have been a fine description for the beginning and ominous enough to plant the seeds of insecurity, while not looking like the villain.
And: "He might be the father, but it is really complicated.", if she doesn't want to do anything bad at all.