So the question is, how can Geralt sleep around with so many women yet romancing both Triss and Yennefer causes them both to dump him?
Well it's pretty simple, its because there is no commitment to all of the other women he sleeps with. With Triss and Yennefer, however, it's not an option as he commits to a permanent relationship with both which results in both getting very angry at him. He never commits to any of the other girls, the closest he gets to crossing the line is with Shani. Geralt actually seems ready to commit to Shani while romancing her, however, she herself decides against it as she remembers how it ended last time. But if it were possible then obviously Triss or Yennefer would not be happy.
However, commitment or not he is still cheating right? So question is do they or do they not know? And if they know do they or do they not care?
Answer is, yes they know and no they don't care.
They just don't, in fact Geralt's cheating is considered a joke by everyone including Triss, Yennefer and even Ciri.
In The Witcher 1 when romancing Triss, she will mention that she is aware that Geralt had other women and that she is not jealous at all.
During "The Great Escape" Geralt questions why Yennefer is not angry at him for going with Ciri to face Imlerith. To which she replies, that if he had died they would not have this conversation and that "the female half of the population of the continent would be drowning in tears".
During "Payback" Geralt will go with Ciri to meet Bea who will be clearly smitten by Geralt and vice-versa, thus forcing Ciri to remind them she is right next to them.
Near the end of the game the lodge will even openly discuss the fact that most of the women present on the ship are ones that slept with Geralt and Philippa will remark that Margarita will soon join the club.
Finally we have to remember that both Triss and Yennefer can and have been reading Geralt's mind, thus if they cared about him sleeping around then they would have dumped him long ago.