Sex is important in a marriage, however
1) It should never be done without consideration for the other party, and
2) It should not be withheld for no good reason.

If for some reason, sex is withheld, then both need to work out the underlying reason. Perhaps it is that time of the month, or she is upset with something you did, or he had a bad time in the office. It is in the interest of the marriage that issues like that be resolved first. The sex will then come quite naturally.

Edit: I just read the TS's post.
In the case of the TS's friend, it may be wise to actually learn how to have sex. There's alot of stuff on the net which will help them better their sex life. The guy should not push the matter, neither should the girl run away from it. There is absolutely nothing to be ashamed about.

Bottomline - Communication is key.
It is very possible to get bored of sex. It happens more often than you think. That said, sex is not the problem. The real problem is -- the relationship. Loving couples can't get bored of sex simply because there's alot of affection going on. If however the relationship is hurt, it is hard to even think of wanting to have sex with your spouse.

The lack of sex in most old couples however, is usually due to a lack of spice/romance in their relationship.