the House prestiges are for always boons, and dependent on the prestige of the Dynasty. The only way you lose prestige in marriage is if you marry your Imperial Princess daughter to a courtier, who would be 4 or 5 noble steps down from her, which is 80-100 prestige loss for HER. This can be compensated if the guy is a Rurikovich, for instance, and his family prestige is huge.
But really, you usually marry for claims and stats (order varies depending on stage of game), prestige from marriage tends to be a RP-consideration