how come most migrants are men?

Many of them are women, too. They just don't get the attention men do. Women travel to other countries to take menial jobs such as domestic help or farm worker. In fact, many rich families have a staff of domestic workers consisting of foreign women.

because men don't get pregnant

That is because ever since our species first evolved, we have been hunter/gatherers. We humans have adapted to a hunting strategy known as persistence hunting. Persistence hunting means the hunter(s) induce prey to spint and run for safety. Then the hunter follows the prey and make it sprint and run again. After several episodes of such sprints, the potential prey will overheat or build up too much lactic acid and become disabled. Because of the need for easier child birth. women have wider pelvises, which make their legs spread out more and as a result, women are not as efficient as runners. Therefore men by necessity had to do most of the hunting. 

When agriculture is invented, the work in the field is also done mostly by men. Therefore when there is overpopulation, it is men who are forced to migrate and look for new land to cultivate to make a living instead of staying at home, where there is enough people to cultivate the fields already. 

Because their real intention is to have sex (consensual or not) with White women.

Looking for pus5y, as usual.

The same reason cave men went off for days to hunt down food while the women stayed and tended the children. The same reasons sailors went off for months on whaling ships. What a strange question! 

Men are able to walk long distances and are able to do work that women can't or don't want to do, especially if they have children to take care of.  Male migrants can work here and send the money they earn to their family back home.  Later, the families can come here and join the men once they are established.