Game mechanism is restricting changes in Diplomatic Relations of realms allowing that each realm can form only single Feteration.
The idea is that under options where Ruler can change Diplomatic Relations with other realms can be added option for marriage proposal which if accepted would immediately define Diplomatic Relations between those two realms as Federation... Federation formed by Diplomatic marriage should not affect ability of the realm to regularly form Federation by Diplomatic Relations with some third realm, and it should be treated by the game mechanism as 2nd Federation (which could be formed only by marriage, keeping the restriction that regularly by Diplomacy only single Federation can be formed).
For two Rulers to be able to propose Diplomatic marriage to another Ruler two conditions have to be satisfied:
1. Both realms must have restricted elections for ruler (Considering that Diplomatic Relations of the realm would be directly related to relations (marriage) between Rulers, it would have no sense to allow such diplomatic option to realms which have only temporary rulers (where rulers are elected on monthly/quarterly elections), but rulers of both realms should be permanent (NO elections for ruler position)).
2. Gender of Ruler to whom diplomatic marriage is proposed must be opposite of gender of one who is proposing the marriage (in feudal society there is absolute no way that persons of the same sex could engage in marriage).
