Actually you still don't get the story. I urge you to genuinely read the arc. When her relationship with Reed did not meet her expectations, she thought their love was over. Namor held out the offer of romantic love to her once, so she went to him when she offered it.
Basically dude she thought they fell out of love and was ready to break the relationship and try things with the first guy who showed her affection. After some soul searching she realizes wow Reed must care and decides to give their marriage another chance.
This shit happens all the time in soap opera stuff. >Yeah, like going on dates with Namor behind Reed's back. Even Uatu wondered what the fuck she was doing.
See its things like this that makes me think you don't read the comics. Just skim through it to find out of context pages.
In this issue Sue is at uni and Namor interrupts her class to have a talk. Namor thinks her marriage is failing again and wants to win her back.
Sue is like nah and is tired of namor's shit. She was just curious to see what Namor wanted.
Reed finds the two of them later and Namor taunts him that their marriage is failing.
BOTH Reed and Sue claim thats ridiculous but Reed decides to beat up Namor anyways for trying to be a wife stealing prick.
Once again mate. Read some fucking comics.