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 main thing you need to decide is whether kleptons are sympathetic characters or "the bad guys".

**If they are sympathetic characters**:

* The vast proportion of kleptons don't actually kidnap human women; this is a negative stereotype, and the few individuals who actually do this are equally disdained by klepton society as they are by human society.
* Some human women may be perfectly willing to have children with kleptons. They care about having a family and want to raise children; their motivation is quite similar to women who adopt or become stepmothers of children who are not biologically their own. (Some or most) other humans either consider these women to be victims or traitors.
* Human women might live amongst kleptons if they are outcasts from human society, either abandoned at birth or rejected for some other reason.
* You presumably chose the name "klepton" to mean "an alien who steals"; instead of having this as the name they use for themselves, make it a slur used by (some or most) humans against them.

**If they are the bad guys**, then they need a motivation for kidnapping human women instead of at least *trying* to find willing partners; and they need a justification within their own moral code for why they think it is OK to kidnap human women. Don't just make it "because they're kleptons, that is what kleptons do, they are inherently evil and have no moral code". Besides the concerns mentioned above, this is also just better writing, it makes them more compelling as antagonists. Then, the way they sustain their population will depend on this motivation, which is getting towards story-writing rather than world-building.