http://tierneylab.blogs.nytimes.com/2009/08/13/do-single-women-seek-attached-men/

"According to a recent poll, most women who engage in mate poaching do not think the attached status of the target played a role in their poaching decision, but our study shows this belief to be false.

Single women in this study were significantly more interested in the target when he was attached. This may be because an attached man has demonstrated his ability to commit and in some ways his qualities have already been ‘‘pre-screened” by another woman."