I am going to sound like a freak when I say this but. I have actually read a study on the effects of sex and athletic preformance.
The study showed that it had absolutely no effect on testoterone levels and therefore had no effect on athletic performance. With the exception of the cardio which was performed. Obviously streneous sex would affect a person just like any other streneous activity. As you body will tire out; therefore taking away you energy when working out.
My advise is to stay away from it, right before a workout. If you are like me you have to work extremely hard for a nut when you are on and it will take your energy.
There is plenty of other time for that. Unless you are an ugly fuck then you should take it when you can get it.