I found this, quite by accident when looking for something else, in a parish bulletin online:
As Catholics, we believe that God has a plan for marriage. Cohabitation before marriage is not part of God’s plan and decreases the chances of success in maintaining a permanent union. Cohabitating couples should separate and live apart before requesting the Sacrament of Matrimony.
I facilitated Marriage Prep. in part of our diocese for 3 or 4 years about 7 or 8 years ago. At that time, about 80 to 90 percent of the approximately 200 couples we worked with on a yearly basis, were living together or sexually active. I’m not sure how many of those couples would abstain for a year to get married in the Catholic Church. Especially, given the fact that a large number of couples were not very positive about having to take marriage prep. in the first place. There was always a certain level of “okay, whatever, we have to do this because my (your) parents want us to get married in the Church”. Fortunately, the evaluations usually came back with comments like, “well, it was better than I thought it was going to be.” But yes, a lot less young people would probably be getting married in the Catholic Church. And that’s an interesting situation because how many of those couples actually stay involved with the Church after marrying. Again, my guess would be that those numbers are quite low. These young couples are getting married in the Church to please their parents and grandparents.
Sadly, this statement done outside of a pastoral setting with Father or Deacon can answer questions about the individual situation (it is not uncommon for cohabitating couples to have a home, children, lives that are as combined as any married couple), I would fear it would discourage people from pursuing marriage.
 For my flesh is meat indeed: and my blood is drink indeed.  He that eateth my flesh, and drinketh my blood, abideth in me, and I in him.  As the living Father hath sent me, and I live by the Father; so he that eateth me, the same also shall live by me.
 Many therefore of his disciples, hearing it, said: This saying is hard, and who can hear it?  But Jesus, knowing in himself, that his disciples murmured at this, said to them: Doth this scandalize you?  If then you shall see the Son of man ascend up where he was before?  It is the spirit that quickeneth: the flesh profiteth nothing. The words that I have spoken to you, are spirit and life…
 And he said: Therefore did I say to you, that no man can come to me, unless it be given him by my Father.  After this many of his disciples went back; and walked no more with him.  Then Jesus said to the twelve: Will you also go away?  And Simon Peter answered him: Lord, to whom shall we go? thou hast the words of eternal life.  And we have believed and have known, that thou art the Christ, the Son of God.
As a non-believer I can’t challenge the first assertion but I can challenge the second. Is there any evidence for the claim that couples who live together before marriage have a decreased chance of a permanent union?
While I’m much too lazy to go get that info, not only is it out there but women, yes particularly women, who have premarital relations are more likely to have failed marriages and the more partners they have the more likely they are to divorce.
This is all information of which I was well aware prior to coming to the Church.
Since so many have sex before marriage a better way to ask the question is for those who wait until married to have intercourse are their marriages more successful. I believe the answer is definitely yes but have not seen the data.
I guess if it really comes down to it, I for one would prefer to see people remain in the Church even if they don’t agree with all of the teachings, don’t live those teachings, and only occasionally make it to Mass, if ever. I would assert at the cost of my own life that nobody who is ever part of the Catholic Church should ever leave and adopt another religion. Remaining a Catholic is the bottom line. Extra ecclesiam nulla salus.
I was thinking just today, if there is any teaching, practice, or commonly held idea within the Church, with which I have a problem, it is this business of the “wheat and tares” — accepting that good and evil can exist side by side. There are churches that never heard of such a thing — they insist that their members live by their religion, not scandalize their fellow brethren, and if they don’t comply, they’re kicked out. I guess, then, “wheat and tares” is the only alternative. Then the only thing to do, is for the Church to nag people to death**, keep reminding them over and over again that these are the Church’s teachings, there is no “wiggle room”, and you are the one that needs to change, not the Church, because the Church isn’t going to.
** - ETA: Perhaps “nag people to death” isn’t the right turn of phrase. It’s not people-friendly and might just drive people away. I meant to imply that the Church should keep teaching the truth about things such as contraception, divorce and illicit “remarriage”, cohabitating, premarital sex, and so on, not just keep silent or mention it only rarely because “people already have their minds made up and there’s no use bringing this stuff up over and over again”. That ends up being “the tail wagging the dog” and reduces Church teaching to irrelevance. People need to hear the truth, and hear it often.
I suppose you would measure the divorce rate of people who did not have premarital sex, with the rate of those who did. I don’t know how you would perform such a study. Anecdotal evidence could also be relevant.
I measure successful marriages as ones where the couple stay together. All marriages have challenges.
Couples who use each other for pleasure before marriage often do not respect each other enough to put in the effort to make the marriage last. IMO!
I’m not advocating for cohabitation before marriage or premarital sex, so please don’t interpret this post that way.
I’m sure some people who live together and/or have premarital sex do use each other selfishly for pleasure, but there are also some who live together and/or have premarital sex who do not merely use each other selfishly for pleasure. There are people who cohabitate before marriage who have awful marriages, and there are people who cohabitate before marriage who have great marriages. And it’s hard to determine if the cohabitation/premarital sex is what is to blame for the problems in the bad marriages. That’s the problem with statistics in general.
I think the Church needs a difference approach to teaching such morals. It seems like a good idea to list all the negative consequences of certain sins, but it could backfire when people don’t always have that experience. For example:
Church teaching: “Don’t live together before marriage because the majority of people who do it get divorced and/or have awful marriages.
Person who wants to cohabitate: “I know three couples who lived together before marriage and they’ve been happily married for sixteen years and have five beautiful children, so…”