This is why the instruction was issued:

"In light of this abundant teaching, the present Instruction does not intend to dwell on all questions in the area of affectivity and sexuality that require an attentive discernment during the entire period of formation. Rather, it contains norms concerning a specific question, made more urgent by the current situation, and that is:  whether to admit to the seminary and to holy orders candidates who have deep-seated homosexual tendencies." 

It's point has to do with the possibility of a man who engages in homosexual acts, has deep-seated homosexual tendencies, or supports the gay lifestyle, to achieve "affective maturity," something necessary for proper exercise of the ministerial priesthood.