Ash and Misty like each other. It's possible that they will eventually marry. According to the story line, they are just 10 years old and there's no romantic relationship between the two of them. For now, they are just two friends deeply into one another.
In the second pokemon movie, Melody kissed Ash and Misty became jealous. This showed that Misty is truly into him. The future series will reveal if they would get married or if nothing of such will happen.
This is the question that people would always like to know the answer to. If Pokemon is more reality-based, then there is a chance that they would get married eventually. There is nothing like having the chance to marry your best friend, right? In the series, however, they are eternally 10 years old. In the society that we live in right now, it is considered taboo when children marry young.
They also do not have a concept of romantic love yet. If Pokemon would make some changes in the future, who knows? For now, Ash and Misty are just a couple of 10-year-olds who are having fun and enjoying their Pokemon adventure.