Who may enter a love relationship with The Iron Bull? You can enter a relationship with The Iron Bull regardless of your sex (he is bisexual), or race.
How to build positive relations with The Iron Bull? You do not need to watch out, while talking to The Iron Bull, in any particular way, because he is very difficult to alienate. The only exception are main quests connected with Templars and rebel mages - in both cases, pick the variant that lets the faction act freely, towards the end of the quest (do not make them join the Inquisition). You can also gain the Bull's favor by killing Venatori cultists and High Dragons with his help. Also, remember to pick all of the dialogue options marked with the heart symbol.
More important steps:
- You need to strive for positive relations with The Iron Bull and wait until you get the first quest connected with him -Demands of the Qun. Complete it as early as possible and, if possible, with The Iron Bull in your party.
- Continue building good relations with the Bull. Some time after you move to Skyhold, he may offer you to accompany him to his chambers. After you spend a night with the Bull, you will have to make an important decision - you can either go intimate with him, on his (quite non-committal) rules or turn him down (this will squander romancing with him).
- During the conversations, you need to make it clear to him that you intend for a serious relationship. This will unlock the abovementioned quest connected with qunari -Tough Love. It is not easy to complete the quest, because you need to defeat a dragon and craft the tooth that it drops, into a necklace for the Bull.
- Giving the present itself is an important moment because, after you spend another night with the Bull, you will be caught by Josephine, Cullen and Cassandra. If you tell them that it was just a fleeting romance, the romance itself will end. You need to make it clear to everyone that you are in a stable relationship (this choice will be rewarded with an additional cutscene).