This isn’t an easy question to answer, as each situation is highly dependent on the circumstances and people involved. In some cases, a large age gap such as this might be acceptable, while in other cases, it may be considered socially unacceptable or even illegal.
For example, if the 18-year-old woman is legally an adult and both people are entirely consensual, then there is nothing wrong with this relationship. However, if the 18-year-old woman is still a minor and if the 46-year-old man is taking advantage of her emotionally, physically, or financially, then this is a highly inappropriate and unethical situation.
Ultimately, it’s up to the individuals involved and their families to decide what is acceptable. If their families approve, then the couple should consider the ethical implications of entering into a relationship with such a big age gap. It would be wise for them both to consider the implications of their actions and ask for advice from a neutral third party.