Romances Where the Couple is Already Together?

I learned today that romance where the couple is already together is called "Family Romance". I'm not doubting what the person is saying at all. I was just curious if these books were popular and where is the actual, I don't know, advertisement or market for it. Again, this really my first hearing about it truthfully.

I'd also be interesed to hear more about stories like this. When I hear the phrase 'Family Romance', it makes me think of 'Mum' and 'Dad' having the ideal family life (two kids, dog, dishwasher, etc.) and still finding the time for being in love. I would so like to prove these thoughts wrong and see that the genre has more to offer: The only story example I can think of where the couple are young, together, without kids and still have a whole story ahead of them without really involving any of the aforementioned things... is 'Beetlejuice'.

Hmm... My story is about a poly triad, where two of the characters are together at the beginning and the third enters the relationship later. Wonder what that'd be called?