Defending Throw-Ins

The question is whether to have a defender mark in front or behind the attacker on a throw-in.

The principle of defending is to mark goalside of the attacker, i.e. between the attacker and the goal you're defending. This is true for throw-ins as well.

If the pass or throw are short, a defender can decide on whether to step around the attacker to intercept, slide-tackle or stay goalside (let attacker receive ball and then delay-jockey-tackle). If the pass/throw is long, then the defender has the closest distance to the ball.

The goal of the attacker is to move into space past the defender, so why give it to them by standing in front of them??

You may want to consider sandwiching the attacker closest to your goal by having a defender behind and in front.