If you told me that sex improves my longevity I would go out and try to sleep with everyone.
Sex does make you healthier but not in the ways you may think.
It has been shown that sex decreases the risk of prostate cancer and heart disease and there is some information that men who have sex three or more times a week are less likely to have a heart attack. For women it increases the hormones and betters the mood. In my opinion it is not the mechanics of sex that make you healthier but the emotional connection to the act that can give you the health benefits.
If you are in a good relationship it helps you much more than the sex. When you sleep beside one your biorhythms are in unison. If you want to improve your health and have a partner, you cannot do it alone. We all know that in many situations. If you want to lose weight, eat healthy, fix you lifestyle, we know that you have to sign your family up as well. In the end, its not about the act, its about the relationship.