In non-Catholic royal families, there were few things worse than marrying a person who was a Catholic. Some countries barred from accession to the throne any person who married a Catholic, as in the British Act of Settlement 1701. When a Protestant prince converted to Catholicism, he risked being disowned by his family, and often being barred from the throne himself.
In non-Catholic aristocratic families, marrying someone who was a Catholic was viewed as highly unfavorable. Some countries had laws that prevented individuals from ascending to the throne if they married a Catholic, such as the British Act of Settlement 1701. When a Protestant prince defected to Catholicism, he risked being rejected by his family and excluded from the throne.