Getting back to the original post, as I understand it we are at least a second generation Vashoth. In order words we have never lived under the rule of the Qun. However, our parents may well have done and escaped from that life for whatever reasons we wish to ascribe to them. Even if the original ones who left were several generations back, family tradition probably hands down where we came from and why we left. That leaves a bit of scope since they might well approve of certain aspects of the Qun philosophy but object to others and have rejected it in its entirety. I guess that bit is going to be up to us. So you are not being forced to follow Qunari culture but you might still adhere to some aspects of it. If you are a mage, and your parents/grandparents were mages, then it is pretty obvious why you wouldn't agree with the Qun.
We are genetically a qunari but not culturally a Qunari. However, the majority of inhabitants of Thedas are not going to know the difference and will respond to your appearance as though you were a member of the Qun.
As for the Qunari attitude to sexual relations. As with everything else, the Qun decides who you should mate with in order to have children. They may well discourage sexual intercourse outside of this officially sanctioned union in case it results in offspring, who obviously will not have been planned. However, I'm assuming that have nothing against displays of affection and forming close bonds with partners of either sex provided it is kept platonic or at the very least you avoid any possibility of having children. You can read into that what you choose.
Iron Bull is currently acting in a way that is totally contrary to the rules of the Qun. However, he is meant to be "under cover" as a spy, so originally he probably convinced himself that what he was doing helped to allay suspicion that he was anything other than a Tal-Vashoth mercenary. Then he found he was being to enjoy his decadent lifestyle and this would appear to be where the conflict has arisen in his loyalties. So it is possible they will allow you to develop a full romance with him that eventually has him choose between you and the Qun, or it may be that he will never be more than a casual fling. That also may be entirely up to the Inquisitor and how they conduct the relationship.