A lot of romance content is dependent on context. How does Cullen join the Inquisition? What happened to him after DA2? What influence does his lyrium addiction have on him? What, if any, personal quests does he have?
If it were me, it would start with him being surprised by the Inquisitor's affections, then initially resistant due to the perception of insubordination. ("I cannot have the men lose faith in me. Our cause is too important.") The topic of his previous romance may come up at this point, with Cullen mentioning that "he had succumbed to this temptation before." Completing the romance requires some re-assurance that it will be all right.
Inq: Love is not a weakness, Cullen.
Cullen: Is it not? Then why do I feel so weak when I am around you?
(Inquisitor sees words will not convince him, kisses Cullen)
This culminates in a discreet meeting at some point in the Inquisitor's quarters. Maybe he comes under the pretext of delivering a report on their military strength or some such.
Inquisitor: Enough. We could have spoken about this in the morning. You came here for another purpose.
Cullen: (sigh) Am I that obvious?
After sexytimes, they talk about how they can't no promises can be made in such an uncertain future, but Cullen makes an oath to stay by the Inquisitor's side to face whatever trials are to come.