The area of moral teaching in which Christians seem to be most hated is the area of sexuality. Persons who act on certain inclinations become angry when told that those sexual acts are evil. Christians are called hateful and judgmental people because they say that certain acts are always wrong. Now, while there can be people who say they are wrong in a grossly inappropriate way (The Westboro Baptists come to mind), the badly expressed message does not take away from the fact that certain acts are wrong.
The anger and hostility directed against Christians is accompanied by some rather bad reasoning. The argument that if people choose to live in a certain way, nobody should tell them otherwise. In other words, we are not allowed to have moral judgments on these activities.
Then these individuals get furious when you employ a reductio ad absurdum and apply that argument to other moral issues which they do find offensive.
That anger is revealing though. It shows that the individual does recognize that certain things are wrong. That individual just refuses to accept that their own behaviors can be part of the group [Things that are morally wrong]. Basically, it means "Other people do evil things, not me."
The problem is, those "other people" can often use the same arguments to justify their own deplorable acts. For example, the same arguments used to justify homosexual "marriage" can also be used to justify incestuous marriage and polygamy: free and mutual consent between people who profess some level of affection for each other. That isn't hypothetical, by the way. I've personally encountered proponents of "polyamory" relationships who get offended when people say it is immoral, using the same arguments the proponents of homosexual relationships – and the same demand not to be judged. These proponents got extremely angry when I pointed out that the concept of "polyamorous marriage" is an oxymoron.
Yet the fact that advocates of homosexual "marriage" do get offended when a Catholic Bishop makes that comparison shows that these advocates do think these things are wrong. So, essentially, that means they believe a moral line exists. Otherwise, when the comparison with polyamory and consensual incestuous relationships came up, they'd probably just shrug and say, "So what?" Their anger then just want to draw the line differently, including their own preference in the list of "acceptable behavior." People don't normally get offended when their beliefs are compared to something which doesn't offend them after all.
But that leads to the question of who draws the line? Either there is a line to be drawn, or else we have no choice but to accept the lowest common denominator. Take Pedophilia for example. NAMBLA argues that the modern age of consent laws are artificial and that consensual relationships can exist between adults and children. Most people find the existence of this group as well as their proposals horrific, but they use the same arguments that are used to justify other sexual activities condemned by Christian moral teaching. They merely add the condition that the age of consent should be lower than 18 and invoke the ancient Greeks and Romans to justify their views.
It also demands a justification for drawing a different line. If people want to say absolutes exist, but want to include homosexual acts as morally acceptable, they need to show how their absolute is justified while other consensual relationships are not. Otherwise they become guilty of what they accuse Christians of: forcing their own view of morality on others. In other words, why draw the line HERE and not THERE?
Actually, when it comes to showing justification for an absolute, Christianity has the advantage – and not from saying "God says so," either. The belief in marriage as a lifelong exclusive relationship between a man and a woman which is open to the possibility of having and raising children has the biological data (pregnancy comes from sex) and sociological data showing the family as the foundation of a society and that societies with strong family ties also tended to have strong societies.
Eliminating the elements of lifelong, exclusive and being open to the possibility of having and raising children makes stopping at "gay marriage" an arbitrary decision by who is in charge. That's the irony of it all. Those people who get offended at this reductio ad absurdum have contradicted themselves. By drawing the line at "gay marriage" but going no further is to make an arbitrary imposition – which is what they have accused Christians of in the traditional view of marriage.
This is the problem with "selective morality." If a person demands that the traditional understanding of marriage be set aside in the case of one preference, then it leads one to ask, "Who has the authority to determine where the law may be redrawn and where the new absolute lies?" Why should it be drawn to only include "homosexual marriage" and not incestuous marriages or polyamorous relationships? Why should the age of consent be 18 instead of 15 or even 11? After all, we can point to a time in the past when such things were seen as acceptable? Of course we can also point to times when slavery and torture and other things we condemn today were considered acceptable, so maybe it's not always such a good idea to point to the morals of the past indiscriminately… it opens up the doors to the reductio ad absurdum.
You can see the problem which those who would change morality tend to avoid. People don't want to permit things they believe are wrong, but they want to justify their own wrong by making themselves the "victim," claiming that the people who say their own vice is wrong are "judgmental" and "hate-filled" people. But then they can't explain why others can't take their same principles and demand to have them applied in a way that goes even further than they want to go.
It's quite clear. Without a rational basis for why a limit should be drawn (a basis Christian moral teaching does possess), one is forced to choose between hypocrisy and the lowest common denominator.