Final answer:
Margaret of Burgundy's support of Lambert Simnel was dangerous due to the threat it posed to the Tudor monarchy's legitimacy and England’s Protestant majority's fear of Catholic rule reverting to religious intolerance. Such support challenged the religious and cultural ethos making the situation perilous.
Step-by-step explanation:
The support of Margaret of Burgundy for Lambert Simnel's claim was particularly dangerous because it posed a significant threat to the Tudor monarchy, which revolved around the legitimacy and stability of the newly established reign of Henry VII. During the late 15th century, England was recovering from the Wars of the Roses, a series of dynastic conflicts, and the crown's authority rested on shaky grounds. Margaret, as a dowager duchess and an aunt to the Yorkist kings, held influence and the means to lend substantial support to pretenders with claims to the English throne, like Simnel, who was posed as Edward Plantagenet, Earl of Warwick, a genuine claimant.
When Parliament opposed the notion of a Catholic monarch, it stood as a reflection of the deep-seated fear among the Protestant majority in England of Catholic rule, which was linked to a potential reversal of religious reforms and resumption of persecution. With England's turbulent history of Protestant-Catholic conflicts, memories of 'Bloody Mary's' reign, and the coercive religious policies of European Catholic monarchs, it spelled out a possible disaster for the religious and civil liberties of English people. The portrayal of Catholics as threats to public safety, perpetuating the horrors of plunder and religious tyranny, resonated with a populace keen to maintain their Protestant identity and freedom from papal authority.
Therefore, Margaret's support of Simnel represented not merely a political manoeuvre, but also a challenge to the prevailing religious and cultural ethos in England, and so was seen as a vehemently dangerous action.