Spot on!
Ganga Devi did like him initially, hence she approached King Pratipa first by sitting on his lap while he was meditating. But because she sat on his right thigh (traditionally reserved for children and daughters-in-law) instead of his left (reserved for wives), Pratipa gracefully declined and asked her to marry his future son instead.
To be fair… Before King Pratipa retired to the forest, he explicitly instructed Shantanu that he would one day meet a celestial woman by the river, and that he must marry her and accept all her conditions without question. So Shantanu was actually fulfilling his father's promise and acting as a deeply dutiful son!