Use the "Add to Collection" icon in the top right corner of any video thumbnail when you're logged in (desktop version of site only) or directly from a video page with the "Add to Collection" button under each video.
@RobbieJM, exactly what I was thinking. It's what I'm thinking everytime I watch porn, actually.
It's a great shame that the safe sex films nearly always edit our the putting on of the condom: this is not only a very sensual thing to watch, it also emphasises the most important aspect of sexual practice. Come on film-makers - do your bit for society and be sexy too!