My ruler's children are getting kids with courtiers?

[Caspian Mortis]

Recently I was conquering Europe as Flanders and my heiress got herself pregnant from an ugly courtier without my permission! She's unmarried because I planned on playing a "virgin empress".
I wanted to imprison and execute her and her bastard child for this insolence but this will displease all my vassels and some of them are already at the verge of rebelling.

Anyway I can do to prevent this, without marrying her to someone?

I also had this happen another time with my male heir so I had him executed, a pity because he had the best stats.

 

[StarSword]

Recently I was conquering Europe as Flanders and my heiress got herself pregnant from an ugly courtier without my permission! She's unmarried because I planned on playing a "virgin empress".
I wanted to imprison and execute her and her bastard child for this insolence but this will displease all my vassels and some of them are already at the verge of rebelling.

Anyway I can do to prevent this, without marrying her to someone?

I also had this happen another time with my male heir so I had him executed, a pity because he had the best stats.

You know, even Elizabeth Tudor is known to have had a number of lovers.

I don't think there's much you can do about it. Even without the Seduction focus, there's a random event with a young adult prince where he impregnates a girl in your court (you can be proud of his virility or force him to adopt the child). Fortunately virginity is of little value in CK2.