i think an important question that has been asked is: does this matter? who cares if Lincoln was gay?
the answer, i think, is yes and no.
we shouldn't care who presidents sleep with, and sexuality should have no bearing on how we view one's performance of his/her job. it's simply incidental to Lincoln the President, and this is simply a footnote -- and an unproven one -- to his legacy, and probably something of a waste of time. to imply that his sexuality may have played a role in the emancipation of the slaves is not only a stretch, but possibly dangerous -- could we point towards a moment of weakness on Lincoln's part (can't think of one at the moment) and say that was due to his sexuality as well? it works both ways; it's simply bad thinking to attribute the positive aspects of one's life and work to a traditionally marginilzed social difference and not also the negative ones. Lincoln was Lincoln, and that's what we should be focused on and 140 year old psychoanalysis is just a sideshow.
however, it does matter for some people -- probably those who are looking to further prove the utter naturalness of homosexuality, and to those who seek to battle it at every point. the fact that Lincoln may have been gay means a lot to that 3-5% of the population who are written out of history, who are given no role models, who are told in a milllion different ways that they are not fully human and cannot participate fully as either citizens or as human beings. to "claim" someone who literally towers over American history is tantamount to finding a new piece of yourself that you didn't know existed, and does much to combat silly stereotypes of gay men as limp-wristed and effeminate, as weak and ineffectual. it also says to everyone -- see, it's not so weird after all. see, it's all throughout history. see, this isn't a post-WW2 creation. see, gay people are everywhere. see, you would have denied ABRAHAM LINCOLN the right to get married and adopt children.
it also matters to those who battle the acceptance of the naturalness of homosexuality because it's both one more piece of evidence that they have to refute and make excuses for as they watch their case crumble, and it simultaneously gives them ammunition to shoot down ANY re-examination of history. it let's them say, those crazy left-wing academics are saying that ABRAHAM LINCOLN, of all people, was gay!!! see, they're all crazy!!! believe not a word they say.
at the end of the day, it remains a theory, and that were Abraham Lincoln alive today, he probably would have identified as gay, and probably would have pushed that well into the closet in order to further his political career. it would be harder to do, today, because we know what homosexuality looks like, we even know what the closeting of homosexuality looks like (pretty much like how Lincoln is described in the review of the book).
could Lincoln have become president today? hard to say, but i'm willing to bet he might have even been elected governer of New Jersey.