Property debt-free - Loan on man?

This is not quite the practice in reality. Of course, there is the joint accrued gains, but even after a separation following 40 years of marriage, the initial assets are still determined and appropriately considered or deducted. The same applies to an inheritance from the parents received during the marriage. I personally find the regular community of accrued gains very fair. (I only find prenuptial agreements important in the case of business foundations and self-employed persons.)

I think it should be designed exactly the same way here as in marriage. There is the current value of the property as of today, and in case of doubt, this must be settled later. Example: The property is worth €100,000 today. In a separation in x years, the man receives €100,000 + x/2, the woman receives €0 + x/2. (x can also be negative: depreciation and prepayment penalty to the bank if one cannot keep the house alone and has to pay out the other.) In the land register then each owns 50%.

This regulation is great, but you would have to somehow notarize it?! And then they come again with the nonsense that this only works if one lends the other the €100K mentioned in the example, which fortunately is not allowed to be interest-free by the tax office... A notary once sent me a proposal, where I nearly fell off my chair... she actually wrote in there that I would have to repay my husband the entire loan amount within a very short time in case of separation... Which can quickly become a problem in case of a loss on the house sale.

This regulation is awesome, but you’d somehow have to notarize that?!
And then they come again with the nonsense that it only works if one gives the other a loan over the 100K mentioned in the example, which fortunately must not be interest-free according to the tax office …

If you get married before the purchase, all this happens 100% automatically, no notary is needed. I also don't understand where you see a 100k loan in the example, because it is not necessary at all in the first place. At most, it would be to avoid gift tax between unmarried partners (I believe 9,000€ in this example). But that would only be a 50k loan for 50% share of the 100k property.

Anyway, when separating, there are two ways:
1. Sell the house: That leaves enough money to pay everyone out. The property equity is inside (ignoring prepayment penalties, devaluation, etc.).
2. One takes over the house. Here the other has to be paid out. But if the separation happens in year 0, it would be 0€ to the woman if the man takes over the house and the loan. And only over time does the woman get (house value - remaining loan - man's initial 100k equity) / 2. Whether that is 50k, 100k, or even 300k depends on the duration and remaining debt. If the man can’t afford that, see point 1.

You should be able to manage this the same way as an unmarried couple, or does anyone know reasons against that?

With a marriage before purchase, all this happens 100% by itself, no notary is needed. I also don’t understand why you see a loan of 100k in the example, because it is initially not necessary at all. At most, to avoid gift tax between unmarried partners (I believe 9,000€ in this example). But it would only be a 50k loan for 50% share of the 100k property.

Anyway, in case of separation, there are two ways:
1. Sell the house: That leaves enough money to pay everyone out. The property is tied up as equity. (Prepayment penalty, loss of value etc. not considered here).
2. One person takes over the house. Here, the other must be paid out. But in separation in year 0 it would be 0€ to the woman if the man takes over the house and the loan. Only over time does the woman get (house value - remaining loan - 100k man’s initial equity) / 2. Whether that is 50k, 100k, or even 300k depends on duration and remaining debt. If the man cannot afford that, see point 1.

As an unmarried couple, that should be achievable as well, or does anyone know reasons against it?

Then it is just a 50k loan … since the man “lends” the woman according to the notary the half of the property … which she would otherwise have had to finance …

As a woman, I would insist on being entered in the land register. Everything else is nonsense.

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This should be possible just as well for an unmarried couple, or does anyone know reasons against it?

Under a civil law partnership (GbR), extensive rights and obligations can be imposed on the persons involved, as long as these do not violate laws and are not immoral. In this case, simply going to a notary helps. A GbR is not "rocket science" and, for example, is also explained in betting pools so that there are no disputes later if a prize is due.

A marriage without a marriage contract automatically results in a community of accrued gains, which was explained in detail above:

With a wedding before purchase, all this happens 100% automatically, no notary is needed.
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In a marriage, a separation of property can also be agreed upon, or even an agreement in between is possible, since separation of property also has disadvantages. Any contract can be adapted to the circumstances.

It also depends on the "willingness to take risks" of an individual which type and content of contract he chooses.

Isn't it possible to somehow have a 60% man - 40% woman share in the land register so that the costs are more or less "equal"?

It doesn't have to be exactly the same down to the cent in the end - I also think that in a relationship/marriage one or the other sometimes spends more. But if it roughly fits, it should be okay.

If I were a woman and paying off the loan and included in the loan, I would also want to be listed in the land register.