I do agree that it's some sort of weird double standard, but I can't exactly figure out why.
For example a hot female teacher goes after a 13 year old boy student, and people think he's lucky.
A male teacher goes after a 13 year old girl student, and no matter how hot he is, people think he's sick, and the girl is a victim.
Why is that? It is because we think the man only wants sex, and is taking advantage of the girl, and that the woman probably cares about the boy? Is it because we think girls that age are not smart enough to know if they want to have sex, and boys are? I don't know. I think it's equally wrong, no matter who does it, but I honestly don't know why people see the situation differently.
Also, I think the idea that the boy is "lucky" is probably not a good thing to perpetuate for boys who might be in that situation. Even though a 13 year old boy might be attracted to his teacher, he still may feel scared and not ready for sex. If we send the message that it's okay, because any male would want to have sex with an attractive woman, he may feel like it's wrong for him to be scared.