Much of Kinsey's work has been discredited. The 10% figure is considered to be on the high end of estimates, with the more likely figure being around 2-3%:
'In the last three elections, the Voter News Service exit poll registered the gay vote between 4 percent and 5 percent. While concluding that the Census 2000 undercounted the total number of gay or lesbian households, for the purposes of this study, we estimate the gay and lesbian population at 5 percent of the total U.S. population over 18 years of age, (209,128,094). This results in an estimated total gay and lesbian population of 10,456,405. A recent study of gay and lesbian voting habits conducted by Harris Interactive determined that 30 percent of gay and lesbian people are living in a committed relationship in the same residence. Using that figure, we suggest that 3,136,921 gay or lesbian people are living in the United States in committed relationships in the same residence. '4
So, if we accept that the data presented by the Human Rights Commission is indeed indicative of the real numbers, then it shows that the census data is only showing up a small percentage of the actual number of gay people living in America. Until, however, a nationwide survey is done, asking questions on sexual attitudes and behaviour, then we can only use data and analyses, such as the ones above, to work out an answer to the question of gay people living in America.
As to the question of marriage, no. Don't see a reason. There are other legal means by which homosexuals can cohabitate and transfer property to each other. Marriage, as ED has pointed out, is by definition a union between man and woman.