You kind of do. People change a lot after being in a major new phase of their life for a year.

I'm not going to say 17 and 20 is outright unethical. It's possible for that to have been a freshman and junior in high school who had a long term relationship. They're not required to break up.

But a 20 year old meeting a 17 year old is significantly more creepy, and is likely due to the power imbalance.