Is it common and ethical to tip an attractive female restaurant server more than anyone else simply because she’s attractive?

Common? Yes. Ethical? I'm not going to call it unethical but I do think the amount of the tip should be based on the level of service received. I don't tip extra just because she's flirting with me.

Ir IS common.  The only thing unethical would be punishing a less attractive person who does a very good job for you. 

It happens all the time.

Probably not fair on the other staff but very common

Just though you tip them 15% for average service and 20% or more for good service and you keep your other reason to yourself that would be okay but do not tell her you are tipping her for her looks or it could be viewed as sexist, which it is but don't let her know it.

seems a bit unfair on the ugly servers. so maybe unethical

No idea, but in my experience, guys that act stereotypically "gay" seem to make the best servers, so I prefer them

Maybe, but it's your money, you're entitled to do as you wish with it.

It's probably common, but not very ethical.

It’s common, but not ethical. What is ethical is to tip extra for excellent service. If she doesn’t deserve it don’t tip her extra.