looking at pornography is an addiction in the same way as drug use and is often much harder to break away from than from drugs.
the reason he looks at things like this is because it is sexually stimulating and he has to expend no effort, time or commitment to a real relationship.
he considers it stress relief.
this is also a viable form of mental and emotional adultery. because he has no responsibility to YOU when he looks at and masturbates to these images, he has no need to develop a real and sustainable relationship to you or the family.
for a lot of people, the idea of viewing pornography is repugnant because it means that they are seeking their satisfaction from outside the marriage.
encourage him to join you in counseling.
there are a few things that you can do right now to try and help the circumstance:
put the computer in the most well used room in the house.
no computer use when both of you are not home/in the same room
time the use of the computer to prevent unnecessary trolling
seek counseling to develop the relationship in your marriage