Christian ideas of sexual morality are based upon standards set down somewhere between 1500 BC and 50 AD. A lot has changed since then, to put it mildly. The Old Testament seems to me to have echoes of a conflict between pastoralists and agriculturalists (see Cain and Abel, and some comments about how Egyptians hated shepherds, for example.) It is also set in a time when patriarchy was just coming into its own, perhaps as a consequence of the transition away from a herding economy to a settled agricultural one. So the standards of morality were based on the need to pass land on from father to son, to have clear lines of patrimony, and to maximize fertility in order to "subdue the land." Women are treated as useful chattel.
Catholic teaching also reflects an overwhelming concern with making sure that people have the maximum number of children. Thus birth control, abortion, masturbation, and homosexuality all seem immoral because they detract from the imperative to have children. (Of course celibacy is a possibility, but actually priestly celibacy was imposed in order to prevent church property from being passed down to sons.)
Interestingly, Protestants and even people with no religious concern at all also had felt a moral imperative to have lots of children until fairly recently. Some American presidents had 8 or 9 or more. It was a sign of wealth and success. It was a sign of manliness to father large broods, and a sign of womanly competence to mother themn to adulthood. People viscerally wanted large families. The moral rules actually coincided with their inner moral sense that childlessness was selfish and immature and irresponsible. People needed children economically.
In the United States in particular (and no doubt Canada and Australia as well) the sense of a large unpeopled land made people uneasy. The idea that wilderness is attractive and appealing is very recent indeed. "A howling wilderness" is how people in the 1700s and 1800s perceived unsettled land. So having as many children as possible made instinctive sense to people.
But clearly both our reality and our inner sense of things have changed. We instinctively know that having more children is not economically necessary or helpful and that the world doesn't need as many more people as possible.
So our feelings about sexual morality no longer coincide with what we read in ancient documents. It takes real mental energy to explain why it's wrong to masturbate or to use birth control. (Protestants only became comfortable with birth control witin the past 60 years or so. In the past everyone knew that only a man having extramarital sex would have need for a condom.) The availability of safe and effective birth control makes pre-marital sex an attractive choice, whereas in the past pre-marital sex meant a child without the necessary support and a disaster for the "unwed mother" economically and thus socially. Now it doesn't mean any such thing.
If the spirit of God were moving among people today, leading them to look for the most loving and considerate answers to questions of sexual morality, would S/He not react to society as it actually is, rather than as it was 3000 years ago? Catholics in their millions have felt led by the most moral of considerations to use birth control and limit their families, and it's only the leadership which is stuck on old mores from old conditions. Most Christians who think much about it see the sense in allowing 2 men who love each other to form a family and adopt children. Surely this is more moral than condemning them to a life of shame and furtive unttached sexual encounters. Most Christians have already thrown in the towel on birth control, and pre-marital sex is condemned only in the faintest of ways. It is obvious to me that the hardline on abortion and homosexual relatinships are a rear-guard action that will whimper to an end within the next 30 or 40 years.
God in times past spoke through the prophets, but today His spirit resides within us, and we know that it is moral to have sex only with a person you are lovingly committed to and to only have as many children as you can provide with a strong start in life, and that they are not workers to help you but dependents whom you must nurture for 20 or more years.
Sensitive and thoughtful unbelievers probably have a keener sense of what today's sexual morality really entails than do people with their heads in an ancient book looking for rules. They hear what the Spirit has to say right now.