Is Sue Sylvester a lesbian?

No she's not.

She got a donor because she didn't have a boyfriend or anything to provide... yeah...

If you remember, earlier on in the season, she was telling her journal that most people assume she is a lesbian, but she did in fact, want love from a man. That was when she started "going out" with Cooter.

no but the person who plays sue (Jane lynch) is

I think she could be classified under the term a narcissistic asexual.

Sure she tried with Cooter, and I'm pretty sure she had "angry sex" with Bryan Ryan (which is hilarious since NPH is gay, and Jane is a Lesbian in real life) but everything always comes back to one thing; herself. She is the center of her own world and any guy is just gonna end up being in the way after a while.