Having actually read "The Merchant of Venice" many times...
I feel that when it comes to text, any text, if it's not written outright, it's open to interpretation. Your teacher is an idiot for choosing not to listen to your theory. While there is no hard evidence that Antonio is a gay man, it's certainly possible.
On a side note, it's interesting to me that the majority of the people who bring up this discussion and believe that Antonio is, in fact, a homosexual, are straight themselves.