I read this article/interview with Sally Rooney today. At one point, the interviewer asks Romney if she thinks the characters in her novels have “healthy relationships” with one any.
[Rooney] pauses. “What does it mean to have a healthy relationship? It’s such a strangely clinical way of talking about interpersonal dynamics, like you can do a white blood cell count and say: ‘No, it’s not looking good for that one.’ It’s impossible to have a loving relationship in which you never cause pain and no pain ever is caused to you.” [...] Relationships, she explains, can never be free from power struggle, and writing a novel full of them is “about being sensitive to how important those power disparities are, but also understanding that it’s not like they exhaust the complete experience of what it means to be a human being or to be with someone else”.
When I read this, I thought it was a great way to describe what Lacan means when he says there is no such thing as a harmonious sexual relationship.
To be clear: What Lacan says in Seminar XX is, “il n'y a pas de rapport sexuel.” This is usually translated into English as “There is no such thing as a sexual relationship.”
However, psychoanalyst Thomas Svolos has pointed out that the word rapport implies a sort of harmony. In English, when we say we have a “good rapport” with someone, that means the relationship does not have problematic tension. 
We can have these sorts of rapport/harmonious relationships with people we don't have an intense emotional attachment to. But after we develop an emotional attachment --when we become emotionally invested, develop a sexual attraction, or fall in love with somebody– things change, and a harmonious/problem-free relationship is no longer possible.
When we are in a relationship with somebody, and that person’s presence, attention, or desire for us, becomes something that brings satisfaction, the absence of these things will bring some level of suffering. Whatever can satisfy us with his presence will frustrate us if it is absent.
That is why I think it is better to translate “il n'y a pas de rapport sexuel” as “ there is no such thing as a harmonious sexual relationship.”
One of the things I like about Rooney’s novels is how they are long case studies that show just how accurate the statement “ there’s no such thing as a harmonious sexual relationship” is.