[[WMG: Sam isn't gay. He's bisexual.]]
At some point in the book, he does mention losing his virginity to a woman. It's debatable whether he told the truth or not, and it doesn't prove much by itself, but...other than his affair with Tracy, what proof do we have that he's gay? His marriage to Rosa didn't work out very well, but that just as easily could have been because Sam had to live with the knowledge that she was in love with his cousin, and that Tommy wasn't really his son. Similarly, Rosa never really got over her love for Joe, and she had to go through their marriage knowing that Sam didn't marry her because he loved her, but because he didn't want Tommy to grow up without a father. For me, at least, this makes Sam and Rosa's relationship a little bit more poignant. Their marriage didn't fall apart because Sam wasn't interested in women, but because neither of them could get over their guilt and conflicting emotions. Also, Creator/MichaelChabon has indicated in a few previous writings that he's bisexual himself, and that he had a few relationships with men before ultimately marrying a woman. When you consider some of the details of Sam's life, it's not that hard to imagine Chabon basing him on himself. At he beginning of the book, he's a nerdy Jewish kid growing up in the big city as an aspiring writer. Based on some of his biographical writings, it's not much of a stretch to say that that described Chabon as a teenager.
* Mostly, Sam's homosexuality comes from his admitting to being a "fairy," as well as the implication of several homosexual affairs with men with names like "Tom and Dick and Harry." (Paraphrase, can't find the section I need.)
* But keep in mind, the story takes place in the 1940s, when homosexuality was still an ''extremely'' taboo subject, and the Kinsey scale hadn't yet been developed. Back then, a lot of men probably would have felt so ashamed of feeling any attraction to the same sex that they would have instinctively considered themselves "fairies".