I’m leaving the harm to the soul apart from this, as it seems that you want to know the “tangible” (quantifiable, measurable, study-able, existential…) harm.
It always start with trivialization - this, by itself, is dangerous.
It is one act, seemingly inoffensive, that we will say “It hurts no one”. What we don’t see is the harm this idea causes to the act of sex as a whole, but we will get there. Moving on.
The couple decides to do some other different things as well. What harm is there in some other practices, after all? There are many a couple can do, with no added harm. At first, they are open to life, finishing the act as it is supposed to; the new activities are only adding some spice to the relationship.
Then, an accident: this one time the man can’t finish in the proper place.
We can discuss philosophy here: did he sin? Didn’t he? He had the intention of finishing in the way that was open to life, after all. Was the act sinful? Wasn’t it?
See how blurred the line already is? How confusing it got?
As it was an accident, he is not responsible by the result…
Or IS he??
After all, he knew the risks. He knew there was a chance he wouldn’t be able to do it right. He sinned not by “missing the shot”, but by putting himself, time and time again in a situation of RISK. That alone is already a sin.
Say, if I were to play Russian Roulette. Even if I don’t shoot myself, I am consciously choosing to put myself at risk, just for the thrills.
Another point by which both are harming themselves: what if they find out they prefer other activities?
He simply loves oral sex, while she finds it demeaning. He might get addicted to that which, while “inoffensive” enough, will be harmful to their relationship.
She might decide that, since other - BETTER - options are open, penetration can be left out. She feels uncomfortable (it happens, many do), and doesn’t want to do it, so both decide to leave that out for now.
When they decide to have babies, she will do it with reluctance, he will do it halfheartedly, if at all; instead of working out their problems with “copulation”, finding out how to better THAT practice, they were too busy having fun. Now, having babies is a chore.
And it all started with a small exception, a dismissing of what sex really is about. Now we have a confused couple, that doesn’t understand when all went wrong. She doesn’t understand why she can’t make him as pleased with anything other than oral sex; he doesn’t understand why she hesitates when he wants to try vaginal sex.
Seems a bit over the top, right? “As if all this would happen!”.
Fact is, many already have this mentality, many are already corrupted by the “trivialization” up there in orange, and many already suffer with all this confusion. So many couples need therapy because “sex life turned boring” (translating: we think sex is about fun, we want MORE fun!) or “he/she won’t do what I like” (translating: sex is about me; pleasure ME!). Most of the ‘options’ (oral, anal, etc) are so self-centered in one of the partners that people have difficulty balancing their ‘times’, of pleasuring their partners. Again, instead of making the one unitive act pleasurable to both, they waste their time practicing acts that focuses on one person more than the other…
TL;dr: trivializing is bad. It makes you think something (sex) is harmless, making you use it carelessly. You end up forgetting what sex is about, redefining its purpose and getting confused in the process. (just like the USA did with the word “marriage” recently)
