Ok, I have limited time, so I'm going to do this as quickly as possible. First off, I'm going to talk a little bit about Larry Kramer and The American People. Larry Kramer is a tremendously important (and tremendously controversial) writer and activist, but he is not a historian. The American People is very much informed by his perspective as a gay man in the 1960's, 70's, and 80's - particularly informed by his experience of the AIDS crisis. The title of the book comes from a speech by Ronald Reagan that referenced "the American people." Kramer knew that Reagan was not including him and other gay folks in that group. Frustrated with the tendency of mainstream history to effectively straight-wash all historical figures, he set out to write a "history" that was inclusive of historical homosexuality.

In regards to the claims he makes in the book about the sexuality of the founding fathers and other important figures in American history, the only answer we really have is "eh, kinda sorta maybe?" Firstly, historical notions of sexuality were just massively different in the past than they are now. The concept of "gay" and "straight" are really very modern, and just don't apply historically. Same-sex sexual behavior was not necessarily considered a marker of difference, or at least not in the same ways. Several figures in American history have had apparent homosexual and/or homoromantic feelings/attitudes/liasons. Whether or not these folks would fall under the category of modern day homosexuality is unclear and up for debate.
All that being said, I think the important thing to remember here is that although it is an anachronism to label these historical figures "gay" it is as much of an anachronism to make a blanket assumption that they were all straight, as we understand the term today. Although Kramer's book is not a history - in the true sense - I do think he does something valuable in his approach. The American People is very much a political work informed by a very particular view point. I think it's pretty important and valuable, but also worth taking with a several large grains of salt.

Larry Kramer is a social activist and not a historian, and he often makes claims that are unsupported. The Guardian did a good piece on his new book with several quotes from historians who say that his book is serving a political agenda, as well as Kramer saying that he uses his "gaydar," which isn't an acceptable way to do academic research. Some historical figures like Washington and Hamilton may well have been gay and simply kept up the appearance of being heterosexual to conform to the social norms of the day, but there's just not the evidence to make the sort of sweeping claims that Kramer does.

Just perusing that article and the Guardian article linked in it I wouldn't take Kramer at face value. He clearly has a political agenda and likes to stir up controversy, and isn't an historian. The publisher is calling it a "novel" Kramer said "It may look like fiction but it's history to me." I think he's ransacking history to search for anything that might support his view. Not only is he trying to place modern ideas and beliefs on people from the last, but his claims are tenuous at best. Abraham Lincoln has had thousands of novels written about them and I am not aware of any that claimed he had a homosexual relationship