Someone living in denial refuses to live in reality because it is too threatening; arguing against a social situation as if the threatening context just doesn't exist. As far as ego defenses go, this one is perhaps one of the most cited in the popular lexicon. We frequently use it to describe the social habits of the drug addict or alcoholic, the criminal, and others who are on the fringe of mainstream society.
In terms of poor boundaries, people of this description might be said to be aggressive toward others, through theirs. We think of violence sometimes in the mental health field when someone has a tendency toward a great amount of denial, because we know that there will be a tendency to ignore the rights of others, and to disrespect them, to take from others or use others because, as one looks out the open hole in their boundary, they simply don’t see the limits of what they control, and instead, feel “entitled” to all that they see outside the boundary, as if they actually control what is out there.
Denial is alleviates anxiety by refusing to acknowledge (or even be aware of) the more unpleasant aspects of a social or romantic situation. In the more aggressive person with poor boundaries, this is commonly seen in those who are persistent, or obsessive about a love interest, who “won’t take ‘no’ for an answer.” It could even lead to stalking behavior. A notorious example from film was the Glenn Close character, in Fatal Attraction.