The only people who have no choice whom they defend are public defenders. These are the people charged with ensuring an accused‘s right to a defense counsel is met. (I assume Mr Weinstein is free to use this service, if he were so inclined and he could not afford or find a lawyer willing to take his case. It appears he is doing fine finding and paying counsel on his own.)
You can make a case, as @twoinanddone has made, that lawyers working for a criminal defense law firm or independent lawyers struggling financially, may be economically compelled to take on clients in order to survive professionally.
I understand that none of these apply to Sullivan. He has freely chosen to represent a celebrity client whose case more or less started a whole new global debate about sexual harassment and assault in the workplace, whose name has become almost synonymous with sexual harassment and assault in his particular industry.
If you are a criminal defense lawyer, you represent no one but yourself, or it could be said, if you are a celebrity criminal defense lawyer, you represent the institution of criminal defense. No one would fault you for representing Weinstein. You are doing your job, just like public defenders. That job will entail casting doubt on the veracity and mental health of every single female witness in the case. Accusing women of lying, of immorality, of fabricating stories about sexual harassment and assault. An unsavoury job, but someone’s got to do it.
If you are the dean of a residential college or house of an educational, you represent that educational institution, and the way that institution organises residential and campus life, and you happen to be part of a national debate about sexual harassment and assault in colleges which is about as vicious and controversial as the debate in the entertainment industry. You must represent the institution‘s impartiality - towards the accuser, toward the accused. You choose to defend the most high profile celebrity accused, you have chosen a side. Perfectly compatible with being a criminal defense attorney. Not compatible with being a house dean.
I am sure there is a clause somewhere in his contract that he has to refrain from behaviour incompatible with being a house dean. Expecting the administration to write „in the case that at some time in the future, a celebrity will be accused of hundreds of cases of sexual assault which starts off a global debate about the matter, none of which any of us anticipates, you may not choose to defend him“ is probably a bit much.