I never really got why..

Because he kissed her too. She thought he was a womanizer. It was a bit trite for her to jump to that conclusion, probably the least authentic part of the movie, but still plausible.

I think jealousy/envy was part of it. She wanted to keep him special just for her, but the thought of her daughter also having him (or him having the daughter the same way as the mother) wasn't something she wanted. Part Jealousy, part Envy. I really don't think it was about the age difference or underage aspect.

LOL because her daughter was 16 and he was 26!!! I would freak out too if not kick his ass!

Besides Sarah and Carter's kiss was sooo...much more romantic...and Lucy kissed Carter, he didn't kiss her ;)

Mr. President Barack H. Obama =)

I think it was a combination of their relative ages, her own age and condition, resentment/repulsion that she had also kissed him, feeling foolish for pushing them together and then resenting it. I didn't think that she was so violently opposed to the idea if he had been a stranger, but given all that was happening, and the fact that he was right across the street, it was too much to deal with and it had to stop.

And while he looked older and cool to Lucy, she knew from their walks that he was an immature mess.

There's also the suggestion, based on Lucy's discussion of the childhood incident, that she would never be able to tell Lucy herself that she objected, and would have to go to Carter to make it end.

or she wanted him all for herself? Just guessing...

Regardless of the fact that she did not like that Lucy and Carter kissed, she did not need to freak out in such a way where her words basically portrayed Carter as a predator. "You will never touch my daughter again!"? Come on, that completely makes Carter out to look like some dirty man preying on underage girls. It was unnecessary and there was never anything done to offset the bitterness of that scene.

As a result, it takes a relatively enjoyable film and derails with that very dark, unpleasant, uncomfortable, and unnecessary scene. It totally ruined the spirit of this film.

most likely it was typical jealousy. Even if she was the daughter, it was as though she felt "old" and passed over for the younger, prettier woman. She used the "she's my daughter and you won;t be seeing her again" excuse because she felt betrayed after all she had shared with carter, including the kiss in the rain...which I thought was tremendously passionate and erotic considering Megs character must of felt so "ugly" knowing her breast would be removed and she might not be attractive to the opposite sex anymore and Carters response made her feel 'desired". It was a good scene, but when the daughter kissed carter, and she saw it, she was hurt.