Often, they're surprised to discover that there are a bunch of underlying issues driving their presenting problem.
Either their compulsive sexual behaviors are creating relationship and other life consequences, or they can't step away from porn long enough to create a life.
They seem to just not care that their actions have hurt your feelings and even your self-esteem.
You could start by saying you are concerned about their sexual activity and their emotional—and perhaps physical—welfare.
You can voice your opinion, set boundaries and offer assistance, but you can't magically make the other person embark on a process of change.
And even if addiction is not the culprit, some type of intervention may be needed.