It's common at airport security checkpoints for a boarding-pass checker or TSA agent to say unequivocally that you can't enter a terminal that isn't your departure terminal when that's actually okay. I think they usually want to be helpful -- don't want to see you miss your flight because you went to the wrong terminal. The best response is a very polite, "Oh, thanks -- I know this isn't my departure terminal. I'm only here to use the [name of] lounge." If that doesn't work, and you really want to go in, you could get firm about it and ask for a supervisor to come over. Maybe the rule at some airports is that you can't do this, but I know it's okay at SFO.