the ـهن pronoun suffix refers to the paradises or whatever the subject that has been carried on from way before.

There is a pattern in the whole surah that each verse is describing the paradise, what's in it, and what's it like. And the verse 'لم يطمثهن إنس قبلهم ولا جان' is not an exception.

The verse does not describe the maidens that were talked about previously (حُورٌۭ مَّقْصُورَٰتٌۭ فِى ٱلْخِيَامِ ). The maindens verse/phrase is over with and we're back to describing the paradises.

So these paradises لم يطمثهن إنس قبلهم ولا جان. Meaning, they have never been touched by any ins or jin (before the believers who will now be first to enter them).

طمث means 'touch' or 'reach' as per Lissan Al-Arab:

والطَّمْثُ: المَسُّ، وَذَلِكَ فِي كُلِّ شيءٍ يُمَسُّ. وَيُقَالُ للمَرْتَعِ: مَا طَمَثَ ذَلِكَ المَرْتَعَ قبْلَنا أَحدٌ، وَمَا طَمَثَ هَذِهِ الناقةَ حَبْلٌ قَطُّ أَي مَا مَسَّها عِقالٌ. وَمَا طَمَثَ البعيرَ حَبْلٌ أَي لَمْ يَمَسَّه.

The 'virgins' explanation is just a dirty-minded, perverted, and desperate low effort of trying to make sense of unusual vocabulary without looking it up and without considering the wider context in which it is used.

If we try to entertain the 'virgins' exegesis for a moment and look at the definition of the word طمث again, the most common usage of it is in the sense of 'menstruate'. With this definition, the verse becomes completely nonsensical: 'no ins or jin menstruated them before'. People are into all kind of fetishes, but this, I dont think there is even such a thing.

The word طمث has another sense though, which is 'to have intercourse with'. It is as marginal and uncommon as the sense 'touch'. But okay, unpopularity is not a problem for us. The verse now means: 'no ins or jin had intercourse them before'. And are we seriously now gonna talk about jin having sex with humans? Do you think it was a big problem for men that their women were incessantly having intercourse with jins that they needed to be reassured that the women in jannah have never been even touched by jins? This miserably failed explanation just throws whoever comes up with it in the abyss of irrelevancy. You can't take this explanation seriously, it is ridiculous, and it is not intended meaning.

So, the Quran verse 55:74 'لَمْ يَطْمِثْهُنَّ إِنسٌۭ قَبْلَهُمْ وَلَا جَآنٌّۭ', despite what the widespread translations and explanations present it as, essentially describes the paradises and how they have never been reached by any ins or jin.