I had a simular problem some time ago, about last year.
Check the source HTML of the page, and see if it has been hacked to include a redirect to the porn site in the head.
The only way I fixed it, (the host was no help), was:-
a) Change hosts
b) Increase password length, strength, to 25 characters, mixed.. That is something like ;j8Tyi786%&^%76t