Oh, it's a complicated issue alright. The nuances and shades of abuse, coersions, control, powerplay, perversion, personality disordedness, and so on, involved in any dodgy relationsip are immensely complicated. But there is nothing complicated about the facts that adults CAN control themselves, and must, and that they know what is acceptable and normal and what is not.
If Forrest found himself mad with love and lust for a pupil, who appeared to reciprocate it, then he had several choices: just control himself, get over it and do nothing. Or leave his job, make sure he had zero contact with her. Or booked himself some counselling ASAP. Or Concentrated on improving his marriage with his adult wife.
None of the choices available to a normal, non-abusive adult include having sex with her and running off to another country (which takes a lot of planning, for those freaks here who are trying to excuse it as some sort of lurve madness), etc., etc.
And can people please try to get their heads round the fact that if someone criticises behaviour such as his, they are not trying to say he is some sort of terrifying, uber-paedo serial killer. 
Most people are capable of realising that there's quite a sliding scale between Normal, Decent Bloke and Fred West.