As someone has said before (but not in this thread), the act of sex the night before does not hurt the next days athletic performance. What hurts the athlete the next day is staying late at the bar, buying drinks for the gal to get her sufficiently liquered up to have sex with him, then making his way home in the wee hours of the morning.
If you've got a steady and want to knock off a quicky, no harm to you the next day. If you're in a strange town and want to go chasing some strange tail, that can hurt you the next day. (and for quite a while, actually, if you're not careful).