When one picks a flower, it does not wilt immediately. It often does not appear to change at all. And yet a change has taken place. Although it has not manifested it yet, the flower will eventually die. It has been uprooted from its source of nutrients and life.
In 1930, the Anglican Church's Lambeth Conference approved the use of contraception in limited circumstances.
This marked the first time in history that any Christian denomination taught that contraception was acceptable for use. Prior to this time, all Christian denominations - Protestant and Catholic - had universally taught that the use of contraception was intrinsically evil.
Over the next 30 years following the Lambeth Conference 1930 decision, most other Protestant denominations followed suit.
But Protestants not only accepted the use of contraceptives. Unwittingly, Protestants also accepted homosexual behavior. How is this the case?
Because ideas matter.
Traditional Christian theology understands marriage to have two primary purposes: the unity of the couple, and the procreation (and education) of children. These are seen to be intrinsic to what marriage is. Thus, marriage must, in principle, seek to bring together husband and wife as one and be open to bring forth children.
Sex, as the deepest and most intimate expression of married love, is the culmination of the marriage into one single act. The whole marriage is, in a sense, contained in each sexual act.
The traditional view says that the unitive function and the procreative function are actually two sides of the same thing. The same act that unites the couple in love is the same act that can procreate. When the man and the woman fully express their fully true maleness and femaleness without reservation (aka the man does not intentionally withhold his semen and the women does not intentionally reject it), their love is not intended to end with them. That same expression of their love is meant to bring forth new people into their love. The couple's love is ultimately not only for themselves, but also for their children.
The unitive and procreative aspects are so linked that you cannot have one fully without the other. The openness to procreation is itself a part of the self-gift which unites the couple. When the couple intentionally changes the act to be closed to procreation, they are no longer giving fully of themselves and their unity is distorted. On the same token, if a couple intentionally procreates with no unity of relationship, the act is degraded to mere mechanics and the educative (of the children) component of the overall procreative purpose of marriage will be hindered.
Now, this does not mean that every sexual act must actually result in a child. The female body is designed such that it is possible for her to conceive at only certain times of her cycle. But every sexual act must, intrinsic to the act intself, be open to the possibility of conceiving a child.
This then rules out all kinds of sexual behavior which intrinsically in themselves are not capable of both leading to unity and of procreating. These include but are not limited to homosexuality, heterosexual anal sex, masturbation, and the use of contraception.
When Protestant denominations decided to allow contraception, they had to fundamentally change their theology of marriage and sex to accommodate this new practice. In order to allow contraception, one has to say that the unitive and procreative aspects of mariage are not intrinsically connected to each other or to what sex is.
Procreation is then separated from what marriage and sex are about.
Marriage and sex are now primarily about the unity of the couple (though it's actually not possible without the procreative openness); having children is a separate decision.
It's not too diffiicult to see how this opened the door to acceptance of homosexual behavior, as well as many other types of behaviors that were once universally considered immoral among Christians.
If sex is intrinsically disassociated from procreation, sex that is not procreative becomes permissible. Anal sex between a man and a woman, as an example, has gained a wide range of acceptance among Protestants.
It's not that far of a jump to go from acceptance of heterosexual anal sex to homosexual sex.
In fact, historically, the term "sodomy", which nowadays has become a derogatory term that refers only to homosexuality, referred to homosexuality as well as heterosexual anal sex, masturbation, and the use of contraception. These were seen as just slightly different versions of the same problem, the problem being that all of these behaviors fundamentally distort the act of sex itself by the participators themselves intentionally changing to act to being not longer open to the normal possibility of procreation.
The flower was picked in the mid-20th century. Although we did not immediately see the acceptance of homosexuality and other anti-procreative sexual practices among Protestants, the door had been opened.
With all the fuss that certain Protestants, especially evangelicals, make over not wanting to accept homosexual behavior, they don't realize that they already did more than half a century ago.
For the Church's position in her own words on marriage, sex, and the family, see:
Humanae Vitae, an encyclical promulgated by Pope Paul VI in 1968 that reaffirms the Church's position on contraception in light of the advent of the Pill:
Marriage: Love and Life in the Divine Plan, a letter published by the US Conference of Catholic Bishops in November of 2009: