sex is probably great jamieyoung...but it is still an activity of energy exertion. better shape = better sex. go swim 4 lap in a pool at the weight anyone ways now.....then lose 15lbs and do it again....those 4 laps will now feel like 1.
sex, much like swimming, is often a full body workout. unless your not into it. take guys for example, loosing stomach helps promote bigger dick, more confidence, etc... being in shape means your body won't tire as easily and you might be able to go 2 rounds or 3 vs. lights out technique (and forgetting to wipe)