How do you enforce it?

As a coach, you enforce your rules by ensuring your athletes have enough trust, faith and belief in you and your rules. A coach shouldn't have to say "If you have sex before the game you don't get to play" or anything like that - the athlete should trust that the coach is giving that instruction for the good of the athlete and/or team's performance.

And if getting a bit of nookie is more important than performing at your best for your team, then you don't belong in that team.

Yeah, it's a cruel world. But top level sport isn't supposed to be nice.
CasperTG

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You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

My Photos:
https://imgur.com/a/80hiRdm