u have answered ur problem in ur own question.
we have told it a hundred times that the people who watch sexy movies or read sexy stuff or visit sexy sites, have a high threshold for sexual stimulation. they subconsiously find the girls seen in that movies, i their wives. but, of course, wife is not as sexy as the girls shown there.(usually wife is very shy also, in initial days of marriage). hence, proper stimulation is not achieved.
also, in early days of marriage, people do not exactly know, what are the things that stimulates and excites them the most. u have to work on that. find out ur and ur wife's likes and dislikes during sexual intercourse and plan ur foreplay plus penetration accordingly.
take ur time and donot hesitate in sharing ur ideas with ur wife regarding sexual pleasure and discuss every major or minor point which is making u worried.
Donot think about the other girls. if u will continue to see sexy movies and thinking about imaginary sex with other girls, it is a possibility that u will not be able to do sex ever with ur wife.