I love the game, but I don't quite understand, how politics work. When I play UE, Militarists get a lot of votes, although I hardly have any military. What can I do to control, which party gets the votes?

Check your population for militarists. Also every system improvement gives influence to a political party, maybe you have alot of military improvements.

you need to check your population screen to check how your population react to events.

for instance a pacifist event may trigger a militaristic response from your population so you have to be careful about how you act.

In the comming updates we will probably have some propaganda project that can be used to increase political support for a party in a specific system