@cksn wrong, it'd be more creepy. Because in society, for the longest time, it was males that had the power. It was viewed for a long time as men getting the girl. From a man earning the right to marry the princess to men being allowed to visit prostitutes and sleep with multiple women and it being viewed as normal.
Also, if something goes wrong, it's sensei that will be pregnant, not the boy. If the teacher was a man and the student was a boy, it'd be bad because not only are females socially in weaker positions for the longest time, but it's the student that gets pregnant.
Now, if the girl/guy is old enough, I don't think it's morally wrong to get together. But I do believe that solely because females can get pregnant, the older male has to have more self control until the girl is ready to be a mom (or at least able to handle being one). On the other hand, with a female teacher, if the boy isn't ready to be a parent, it doesn't matter as much because she can care for it herself. She'd be the one that's pregnant.
Besides, a boy wanting to bang a sexy older woman is one of the most normal things as well. In fact, it's more normal because if it happens he could brag about it and he himself wouldn't be the one to suffer the consequences.
Of course, even then, it's actually modern society that's weird in regards to how we deal with teen romance with older people. For most of human history, teenagers were treated as adults. Young adults, but still.
It's literally the age where people are ready to breed. Our bodies are in a state of wanting to breed, yet we are viewed as children.