Adding a female perspective..

Yes. Sex tapers off. It happens slowly,and you won't be prepared for it. Multiple reasons behind it, but it all boils down to one thing. The female is in control of it. You can be the alpha male and be the sole breadwinner, handling all the finances, the scheduling, etc...but she will be the one that controls the sex life. That's called ammunition. And what do we know about ammunition? It's used for fights. Then it becomes a positive/negative reinforcement for good/bad behavior. Then you'll never have sex because she is overcome with passion, just because you vacuumed the living room.

I would say that the whole, "We're just too busy" is valid, too. People in relationships easily become roommates with benefits if one or both have a busy schedule. When couples are busy, it's not easy to fit sex into that schedule. They may both love one another very much, but I'm sure you can imagine how unsexy it is to look at one another and say, "I've got a conference call in five minutes, wanna knock boots real quick?".

Passion tapers, sex tapers. Happy relationships still prevail.

Edit to add (if you feel this is relevant): I've been in a relationship for 7 years. We have kids. I work full time. He's a professional couch potato:) I'm still madly in love with him.