Sex as an Act of Worship?

Every once in a while I'll hear a pastor give a sermon in which he (only male pastors have mentioned this thus far in my experience) will mention that sex is an act of worship. I've never heard this idea expanded upon, and a quick google search failed to turn up anything of interest.

In my marriage, sex has been used for many different purposes. Pleasure is the most common one, of course, but other reasons for it have included: comfort, bonding, relief of stress or anxiety, a release of sexual energy, etc etc.

I don't know that we've ever used it as an act of worship, though. I'm not even sure how one would go about doing so, and would be curious to hear anyone's experiences with it.