Lovingly.
Have they been under stress? Very focused at work? New changes in your lives?
The ebb and flow of sex in relationships is normal. You got together, it was awesome, exciting, hot and heavy and now you are in a lull reflecting back. All normal behavior. You are not wrong for feeling that way and you should address it head on.
As this is a touchy subject, again voice your concerns with respect and understanding.
9 times out of 10, they will appreciate you speaking to them in confidence, as opposed to cheating or telling the world, your family and friends about the personal aspects of your sex life. This should encourage them to take what you are saying to heart and seek a change.
Find out what may have changed things and explain that you'd like to focus more on your sex life and intimacy.
If nothing changes and it's a deal breaker for you, communicate that. Honesty and not being afraid to discuss tough issues, solves almost any problem ever created.
Questions/Comments below!