Communicating via the Internet is no different from sitting across the table in your local coffee shop and having a chat. The distinction is spurious. Before people sat staring at their phones/tablets/laptops, they sat daydreaming, thinking about someone they’d spoken with earlier in the day or wishing they knew someone they could talk to about something.
The real issue is that many people aren’t really being conscious about themselves or their relationships. And frankly, if you can’t turn off your phone and spend time with the people who are around you, you’re in trouble. If you think your partner chatting online with someone else about something you two don’t typically discuss is “boundary crossing” then you’re in trouble. No one person can be everything for any other person. We all have different interests, different privacy needs, different dreams and hopes. Giving each other space and freedom to grow is essential to a healthy relationship. Self-censorship and fear are antithetical to a satisfying long-term partnership.