Question: The male klepton is a unique humanoid. It has no females, and reproduces by mating with other humanoid females, who bear children that are clones of the klepton father. No females are born, and all offspring belong to the klepton species. While a settlement of these beings could simply obtain the necessary mates through kidnapping, this isn't exactly sustainable; raiding cities to enslave their women (or anyone else) isn't exactly good for diplomacy

How could this humanoid maintain a settlement, where most of the population is of the klepton species and the children end up with their fathers, without the need for any kidnapping or sex-slavery?

Answer: 
**Animal mother.**

As you set it up, the klepton DNA evicts DNA belonging to the egg of the mother. Or more likely the klepton "males" actually are neither male nor female, but mitotically produce a single cell clone of themselves that has the ability to parasitize the reproductive tract of a female. A gamete like a sperm is a product of meiosis and cannot give rise to a clone of the parent because meiosis shuffles the genes. The kleptons are male only in that they use a penis to place the zygote in the host female.

There is no need for the female to be humanoid or sentient to participate in this endeavor. She does not contribute DNA. She only needs to be the right size to gestate the klepton. A large domestic animal would be fine in this role.
The humanoid females don't need to be married forever; they can simply stay until the child(ren) they bore are fully developed, then they can freely go back to their places of origin to seek new mates if they so please (if they want to ensure their own DNA gets passed down).

If this is a klepton species that's closely related to other humanoid species, I think that the more likely mechanism isn't the children being clones of the males; rather, during the process of gametogenesis, the klepton species' gonads discard the portion of the genome inherited from their mothers. A similar mechanism usually goes into play among the most well-known examples of klepton species. This ensures enough genetic variation to keep the species healthy while also maintaining its distinctness.