Q : When do you know a mobile platform is successful ?
A : When the platform starts to attract adult apps ..
Microsoft has said NO to porn / adult content ( but there are a few lewd looking app icons on there ) on the marketplace but that has not stopped a developer Devil-Y Software from releasing porn / adult apps for the platform. The developer has released a client for tube8 which brings adult video content to Windows Phone called Tube8 and if you are after images there is an app called PixChan7 that brings xxx images to your windows phone. You will need to have an unlocked windows phone to sideload these apps on to your device.
Tube8 (via) offers a trial mode which allows you to stream content but should you feel the need to download the video to your device or add password protection to the app you will need to spend $9.99 and get the full version ( NOTE : I have not tested the full version so this is just the info provided by the apps description on the devs site ) . PixChan7 (via) is a free app so you have access to the full feature list ( which is just like any other image viewer but offers adult content ).
Find free adult apps for Windows Phone here.