Eugene Volokh has some interesting numbers on Americans' attitudes toward homosexuality over time and in different age groups. A few of his observations are noteworthy. Over half of Americans are still opposed to any homosexual sex as immoral. I wouldn't have guessed that it was over half, but then again I've lived in blue states my entire life. The most interesting element is that there doesn't seem to be much of a shift in attitudes within each generation over time. In some age groups there's no statistical shift. That means that exposure to gay people and portrayals of gay people in the media doesn't change people's attitudes to whether homosexual sex is wrong, even if it will lead to people's being more accepting of gay people, as it almost surely does. This also shows that a large enough percentage of the population must be able to consider gay sex wrong and yet accept gay people. I wonder if these distinctions are easier for people to make if they happen to think gay sex is wrong than they are by those who don't. In my experience, it's the latter group that seems unable to consider the possibility that anyone could take both of those stances, even though a large percentage of Americans do exactly that.
One of Eugene's conclusions is less sure but very interesting. There seems to be a bigger change in these attitudes in the generation that grew up before 1973. That means this isn't at all a result of the gay rights movement but is more likely a direct result of the sexual revolution. Those who were young during that movement are the first generation to approve more of homosexual sex. It's true that later factors explain why the next generation is more approving than the previous one, but the big change was simply the sexual revolution, and then those raised in that period communicated their values to those in the next generation effectively enough that the next generation had more permissive attitudes.
What becomes clear in the comments is that the 55% figure for who thinks gay sex is wrong is a marked contrast to the 80% of Americans who associate themselves with the major theistic religions. Now it's true that some of those people are involved with liberal religious groups that don't take their scriptures seriously enough to accept what they say on this issuue, but I don't think those people are over 25% of the population (some of the 55% are surely non-religious to begin with, though I suspect not many, but that would mean more than 25% would associate with the main religions while approving of gay sex). I don't know the numbers on all those. Does this make more sense if you do know them, or is there an alarmingly high percentage of Americans who say they follow a religion but don't really believe what that religiohn teaches?