On the market for rentals, gender and ethnicity is, by law, protected from discrimination. You can therefore not choose or reject a tenant based solely on their nationality or gender. Likewise, you cannot write in your ad, the you prefer (or do not prefer) a certain gender or nationality.
The above does not apply, if the lease is part of your private home – for instance, if you are renting out a room. In that case, you are free to choose whether you prefer a male or female tenant, and you are also allowed to write it in the ad.