Margarie did what she had to and while there was some sincerity, don't read too much in what she told Tommen such as "I'm yours forever." She's actually smarter and more manipulative than her granny as the granny snared her husband with her body, while Margarie snared hers with her words.
That, in reality, could well be how it is forever with Margarie rarely having to use her body with Tommen unless it's necessary i.e. covering up a pregnancy through an affair, of which I'm sure she'll have many. Whereas Tommen will stay loyal to his wife, he's the opposite of his "father" Robert basically.
He's a nice kid who'll grow up to be a man easily manipulated by all around him. It's sad in a lot of ways, but I suppose being in any way soft and naive around here means you get what you deserve.
But Margaery's so hypnotic. The way she does stuff. Even Olenna claimed that Margaery was better.
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