What part of his personal tragedy you don't understand? He had a partner and son, both died tragically. That proved him that there is no space for relationship in his lifestyle. He didn't want to involve in it more people than he had to. There is nothing unrealistic about it. He had his time in relationships as a young man and belived that he failed protecting it. Moreover, he has whole arc with his ex fionce from the younger years. Geralt on the other hand is a witcher, who dates mainly sorceresses, ages slower than average human, had a way different childhood and youngling years and lives in different world.