Mate you are talking out of your arse like some elitist moron. There are no "objective opinions" the same way that there is no objectively right way to write a story. What does exist is a manner of how authors of the genre usually write stories - conventions, habits, distinct features. And even then you would be hard-pressed to find an example of a story that does *not* mix genres or deviate in other smaller or larger ways. Obviously, the departure from conventions may not always result in something surpassing the original, but it is, in essence, what moves literature forward and makes things interesting.
In my eyes, what you want is akin to a cookie-cutter template with exceedingly little variation for no other reason than genre-related "purity". The reason I am saying this is precisely because you mentioned "the first penetration", which you claim is "perhaps the most important" - that, in and of itself, is a distinct stereotypical feature of the genre. However, there are absolutely no rules that specify how and where this has to take place. I, for one, enjoy the author's decision to go against the usual flow - that is nothing more than another opinion, though.
When it comes to the "believability": I agree that the scene needs some more descriptive elements that make the situation at least slightly more likely in the context of suspension of disbelief. As Jebulus so crudely put it a few posts above, there are quite a number of factors that make the FMC not noticing anything unbelievable. The solution would be to add more descriptive little tidbits that paint her condition in that moment in a way that makes it*more* likely, such as extreme sleepiness after waking up from a nap which causes her not to think much of the things that she notices are off with the situation. Maybe she also woke up from a wet dream, that would solve another issue regarding the logistics of it all.
Lastly, disregarding the immediate buildup, the scene itself is really good because it is rather unique. The whole "legs like a baby fawn due to overwhelming pleasure preventing her from getting up" combined with the accidental nature and early-corruption unwillingness of it is something I have only seen one other time, as far as I can remember (Hangover Cat). Granted, that particular work does not take itself very seriously, but in concept, I believe the author can make it work with some more plausible elaboration. At the end of the day, it will likely remain as a rather more unbelievable scene (relatively speaking), but that is not bad in itself. What counts is what the author makes of it in the following scenes - and from what I could see from 0.5, he handled it about as well as possible and did not blow too many "first times", if you know what I mean. One thing I will mention though is that it is unclear in the last few moments of that particular scene whether the penetration is still surface-level or if he goes deep - I would prefer the former as that would save one more "first time" for another scene, and they are handy resources to have when it comes to building tension, as others have mentioned.
Fkn hell, this was much more than I intended to write, but w/e, rant over.
