Hi Geri! Very interesting question here! The correct way is saying: No me di cuenta DE que eran novios (see http://www.fundeu.es/recomendacion/darse-cuenta-de-que-no-darse-cuenta-que-988/). I would then say it is correct to use both tenses in the past (so: me di cuenta, and eran novios) because of the rule of "concordancia temporal" (the latin "consecutio temporum" applied to Spanish). So, if the verb of the main tense is in "pretérito indefinido", like your "me di cuenta", the second part of the sentence, which happens simultaneously, must also have the verb in the past: in this case, "pretérito imperfecto". It doesn't matter if the action of the subordinate is still going on now: "ellos son novios a día de hoy". For example: Me di cuenta de que era gay (even if obviously I still am gay). I hope it's clear :)