Legacy tree should be expanded to showed your married companion as your wife/husband. Would also like to see more interaction from them. Have them greet u when u board your ship. Send u more mail. Spend all your credits shopping.
They didn't put it in the legacy tree since you could have several characters 'married' to the same companion, which would be awkward.
They missed the boat on companion interactions, that's for sure. I hate how I can't even talk to them anymore after you finish their quests. I don't see any reason you can't have some kind of system to create recurring 'companion' quests where at least you can talk to them.