She keeps doing it into Act 3 because (For all intents and purposes) Hawke had agreed to an Open Relationship. Isabela said she had no interest in love or really attachment. That was their relationship up until the point where she admitted to loving Hawke.
The morality of our world still very much applies. It just wasn't the relationship you think it was.
What in the maker's name is an open relationship? Friends with benefits? That is not exactly a "relationship".
Or did they wanted to show a free woman, who doesn't want to get tied down. Her running away from Hawke would have served that purpose. She didn't need to sleep with half of Kirkwall to prove it. Or at least have the courtesy to not do it front of hawke.
The only normal companion was Bethany. She didn't whine or cry or had hidden agenda or was completely crazy. She loves her family, is religious but not a fanatic. Her view towards magic is pretty neutral. She understands how much difficult it was for her family to protect her.
And then she either dies, becomes a warned or gets sent to circle.
The worst part is hawke doesn't even go to meet her once in the circle. What an ass!