Is it permissible to marry another girl for sexual pleasure, while fearing that the first wife may ask for divorce and obtain custody of the child?

You must repent to Allah for looking at what is not permissible for you, and fear Allah and limit your gaze to your wife. There is no harm for you in marrying another woman who is more beautiful if you are able to be just between them. In the event of a divorce, the mother has the primary right to custody unless there is an impediment.