You need to come out and tell them you're straight, and no lesbian...if you don't communicate then it will continue.
If it bothers you I guess you'll just have to keep telling them that you're not. Outside of that I don't know how else you would go about it.
Some people take your words and turn it into "thou dost protest too much". It's hard to know your situation, but if it's not a big deal to you, just let it be.
Or, you could take the approach most considered "cold." Just don't care what anyone says, like I do. If you don't give them the satisfaction of bothering you, it will subside.