Bad thought

At least he's supportive just in case you ever find that you are in fact a lesbian.
It's good to know that you have family who'll support you no matter what.

So tell them you're not. I'm not really getting the problem here. Atleast you know they'd support you if you were. You should appreciate that.

It is nice they are supportive of you and trying to find you someone. There are far too many who reject children for being gay (even if they are wrong in their assessment that you are a lesbian).

At least you can still be friends. If you want to add extra benefits down the line, that discussion can happen later on.