This is a great, very thoughtful and thought-provoking 2009 article about why so many sex-positive "progressive" people like to associate sex with "spirituality". It's written from a skeptical perspective, so it instantly jibed well with me.
One thought from that article is that "spirituality" is pumped up with positive associations because it's a close neighbor to religion, and religion is, in our minds, still stereotypically "good". So if you move out of the "sex is sin" paradigm, and into a "sex is goooooood" state, then calling your sex spiritual makes the transition a lot easier. It alleviates your guilt, simple as that.
Personally, I would like to add that "sacred sex" or even just plain VERY VERY GOOD SEX has a lot to do with introspection, and "spiritual" practices are, of course, deeply introspective most of the time. So I guess it makes sense for many people to combine the two in order to achieve even better results, or simply because there is a huge shared area.