I think that from the amount of writing both on these forums and in books that this issue is not clear to many people. There are many people who want laws that are clear and decisive along the lines of “thou shall not murder”. Those people, like myself, are often left wanting in these situations because there is no CCC reference that is a list of prohibited and allowed sexual acts.
Yes, people like us are “Letter of the Law” type people. Yes, we often don’t understand “Spirit of the Law” mentality. To us if it was intended as part of the spirit it would be written
clearly as part of the law. I really think that this mentality difference is what causes much of the “debate” on this forum.
In the OPs case, he asked if he sinned. The sin in this case, if it existed, would have been Lust, the separation of sexual pleasure from both procreative and unitive aspects of the marital act. My personal educated opinion is that he and his wife clearly
DID NOT sin. The act was unitive, up until the interruption. There was genital to genital contact, which no one on this thread has bothered to post actual research on so I will. Please be aware that this page is graphic:
en.wikipedia.org/wiki/Pre-ejaculate. This Wiki page makes it clear that many of the studies that found no sperm or no motile sperm were small groups and not done with the best of the scientific controls. It also seems very clear to me from a bit of research that men vary
wildly in sperm counts which also effects their pre-ejaculate sperm counts. With that said I think it is safe to say that the evidence is mixed and we can not say beyond a reasonable doubt that withdrawing before climax can ensure no pregnancy. Therefore while the procreative aspect was severely diminished, it may still have existed. Doing so normally would, I believe, be disordered however there are additional issues in a case such as this.
Finally, we need to remember that free will is required for a sin to be mortal, and while we would all like to to be free of all sin I suspect our OP is concerned with a Mortal Sin in this case. There are three impediments to free will here. The first is a bottle of wine which may have impaired the couple just enough to allow things to get started, the second is that it was clearly a peak time which means a large spike in hormone and pheromone levels that each spouse is exposed to, finally it seems very clear to me that the OP engaged in intercourse not because he wanted to, but because he felt he was required to in order to avoid a sin.
Having been in the same position as the OP in the past, I can tell you that while I love my wife, the pressure to avoid impropriety is strong and there is little of the true feeling of love left. What is left is a desire to climax and feeling that I must climax to make this legitimate. To me this occurrence now separates the unitive aspect of the marital act from the procreative in the opposite way that we normal think of.
We put the pressure on ourselves to uphold the procreative aspect of the marital act so much that it now dominates the Unitive and Pleasurable aspects of it.
Unfortunately, this is something I struggle with to this day. How do I deny my love for my wife and our desire to experience the highest unitive act we can while maintaining the procreative aspect when we can not risk another pregnancy (for medical and financial reasons).