Through the awesome powers of Evolution.
All the men who didn't survive died. Those who did progressively became more successful at it overtime.
As to why a Dragon wants to impregnate a man, perhaps its the particular hormonal cocktail that men just have. Maybe its that Women simply die. Pretty gruesome stuff really. Maybe dragons just happen to be very open minded individuals that just tried everything too slow to escape - it being the simple law of numbers that male humans happen to work.
Now fast forward several tens of thousands of years. Dragons have been using male humans for so long now that they are co-dependent on them. On the other side of the stick it really helps to have a Dragon around particularly when there a so many predators that think slow humans are a great afternoon bite.
Then the Humans just had to upset the apple-cart by getting smarter, much smarter. Now Humans and Dragons are in an Arms race for intelligence. Eventually this arms race settles down into a cold stalemate with both Humans, and Dragons being intelligent enough to create societies.
Now here is where we get into the weeds. We want a society that places Dragons on an equal enough footing with Humans. If after copulating the human re-enacts the scene from Alien where it bursts out of them in a bloody killing spree this society will never exist. The process can be bloody, but it needs to be "natural" as these people would put it.
This could be achieved intellectually. If there happens to be a plethora of species on this planet practicing a similar reproductive strategy. Perhaps Big Cats, like to get it on with male badgers? Unfortunately you are already playing the odds game as it is. This world would be the 1 in a billion worlds that even have this reproductive strategy.
More likely, somewhere in those tens of thousands of years Men must have developed a less messy, and less lethal method of delivery. In all likelyhood Men are in the process of developing a distinct "Womb-like" organ. It wouldn't necessarily be a placenta, it could simply be an Egg carrying sack. Perhaps it has a controllable level of blood flooding. Or it might be more advanced with a placentalike organ. These have evolved several times over the millennia, so not completely out there but really do need deep time to get right.
Of course the "development" might not be on the human end, it might be on the "Dragon" end with the egg producing hormones for manipulating the male body in just the right way.
Now there are two larger problems with this scenario:
Hybridisation. Unless its two dragons getting it on, and the human male is a nurse maid. Human genetic material is intermixed with Dragon genetic material. Hybrids are rarely capable of pro-creation, and often have numerous genetic diseases. Deep time would eventually iron these out, but tens of thousands of years is not enough. And there is already a highly effective strategy for reproduction: getting it on within your own species.
Male looses its definition in this context. They may call themselves male, but these people have essentially become hermaphrodites. They are male in one context, and female in the other. This is a problem because you have stated clearly that this is a "cheap shot". Unfortunately the alternative is inline with the other answers, and his reaction wouldn't be "come here darling!", but "get away from me you predator!".