"Europe" is rather larger and more diverse than Americans sometimes think - medical practice, and medicine availability, is different in the UK than in France, and different again in the Netherlands, and so on. As explained on the website of the European Medicines Agency:
While the majority of new, innovative medicines are evaluated by EMA and authorised by the European Commission in order to be marketed in the EU, most generic medicines and medicines available without a prescription are assessed and authorised at national level in the EU.
Aside from that, of course, about half of the countries geographically located in Europe are not members of the EU. That makes it hard to rule out that there is somewhere in Europe where it is prescribed or marketed as such, unless we can find a more specific claim - we would have to investigate its licensing in as many as 50 countries, depending on how you define "Europe", in order to conclusively prove the negative.
Searching the EMA website, I found two EU-wide registrations for melatonin:
- Circadin, authorised in 2007 for treatment of insomnia in patients aged 55 years or over
- Slenyto, authorised in 2018 for treatment of insomnia in children with Autism Spectrum Disorder or Smith-Magenis syndrome
There are some applications for treating other conditions, but none related to fertility.
Neither application mentions fertility problems as a known side-effect, although I did find the following in the Risk Management Plan Summary for Slenyto:
Important Potential Risk: Delay of sexual maturation and development
Evidence for Linking the Risk to the Medicine: The active substance is the hormone melatonin which regulates the reproductive process in seasonal but not in continuous breeders; however, it delays sexual
development in the rat in a transient and reversible manner (Lang, 1986; Lang et al, 1984).
Risk Factors and Risk Groups: Children and adolescents that are between pre-pubertal to pubertal stages.
It recommends that this potential risk be monitored and studied further. The connection between melatonin and fertility therefore doesn't seem to be completely without basis, but the gap between that and active use as a contraceptive seems implausibly large.
Similarly, the UK National Health Service website has a page on melatonin describing its prescription for sleep disorders. In the Q&A section, it says:
Will it affect my fertility?
There's no firm evidence to suggest that taking melatonin will reduce fertility in either men or women.
But speak to a pharmacist or doctor if you're trying to get pregnant. They may want to review your treatment.
Will it affect my contraception or HRT?
Melatonin does not affect how contraception works, including the combined pill and emergency contraception.
However, your levels of melatonin can increase when taking the combined pill or hormone replacement therapy (HRT).
Speak to a doctor or pharmacist before taking melatonin if this applies to you.
So, again, contraception is not even an expected side-effect, let alone an encouraged use.