What does it mean when a guy says he can't imagine me being a lesbian?

In all honesty, I don't know what that's supposed to mean or how it even came up? Why did he say it, how was it relevant?

Maybe he's speaking from stereotypes and saying, "You're too feminine or pretty to be a lesbian" - That's my honest best guess. If he's speaking like that, to you, it's a compliment...but an offensive presumption about lesbians.

I'm not really sure myself haha! Well maybe you don't fit the 'stereotype' of a lesbian, even though lesbian women come in all shapes and sizes, he may not see it that way. Or maybe you are just very successful in being with the males! Anyway, I wouldn't really take it as a compliment or an insult. He is simply saying that he couldn't see you as a different sort of person. Lesbians and straight girls are both the same, so I wouldn't take offence to it.

I think he's referring to your innate love of penis, and you probably love it so much that he couldn't imagine you loving vagina. Because you love penis so much. I don't know, ask him?

It doesn't reflect on you but on him.