I'm 20, totally in the closet so I'm in your boat..Granted I think I am bi because I can and have hooked up with girls, I find them attractive...I don't think me and my friends have ever gotten that graphic lol ...besides like "Oh god I want to Fuck her" or "Look at her I just wanna bend her over" or something
I guess you could just agree or make comments like "Oh god, so do I" or "That'd be sick/chill/awesome" or something.
Or if you don't want to lie just say vague things like "Oh I'd do things you wouldn't even imagine." ...they don't have to know that the things would be going shopping, gossiping and such
jk