Thank You very much! If they have the "legal" ability to marry then that is really all that matters. IF they fail to follow the rules and protocols to retain their "Royal" status and privileges then that is on them, as they have no "right" to them otherwise, imo.
Their titles and styles are enjoyed as a consequence of their precedence and place, however, they can be granted or withheld by The Sovereign at any time and for any reason.
So, they may legally be able to marry anybody they want, but if they defy the Crown, the price would be the loss of their royal rank and title.