We usually think of abuse as obvious, perpetrated by a tyrant where the effects are visible. But there are many subtle forms that cause damage while masking themselves as something else. All abuse, regardless of the form it takes, is about instilling doubt in order to maintain control. If you can keep this definition in the front of your mind confusing aspects of relationships start to make sense.
One way that abuse masks itself is when some topic is generally known to be off-limits. Exploring it might make a person sad, upset, or anxiety ridden and you both end up never going near it. This is a pretty nifty trick because you get turned into the villain for having the audacity to broach the subject while the person who is actually instilling doubt and maintaining control over what is okay to talk about appears to be the victim.
At bottom this is really a protective maneuver that has to do with self-preservation. By having the power to sanction what can be talked about and what must be avoided anxiety is lowered and the threat of the relationship changing its current form seems to go away. What ends up happening though is that the attitude wreaks havoc and creates a lot of unconscious hostility. As a general rule, in healthy relationships there is no topic of conversation or area of inquiry that is off-limits. When both people know they are on an equal footing, have equal rights, and respect and care for each other, some topics might seem dangerous but they play themselves out in a safe space.
By remembering that abuse is instilling doubt in order to maintain control you can raise your conscious awareness when venturing into areas that are implicitly or explicitly off-limits to insure that you are exploring these areas in a respectful way that recognizes the emotional and psychological rights of the other. Discussing them openly is usually essential to the health and growth of a relationship even though the conversation seems scary or abusive. True abuse is a situation where one person has all the control over what can be talked about.