Even though files at xvideos.com have .MP4 extension, they can be captured the same way that .FLV files are captured from Adobe Flash Player. You can play them with Media Player Classic if you have K-Lite Mega Codec Pack installed.

I didn't have any problem recording video streams from xvideos.com using Replay Media Catcher and Bit Recorder

Please read the following FAQ to learn more about recording/downloading/saving/capturing/ripping http:// .flv video streams:
How to record/download/capture/save Macromedia/Adobe Flash .FLV video HTTP stream from embedded player
How to capture adult/sex/porn streaming videos from XVideos with Replay Media Catcher (tutorial)