I don't think it's at here expense at all. He's talking about having fun with her.
And Ryan, I don't think you're under any obligation to disclose that you're trans until the contents of your pants become an issue. But since it sounds like you're really young (young enough that "my voice hasn't dropped" sounds plausible), and since relationships can move more slowly in that age range, it's possible she could get emotionally attached to you before the "pants rule" comes into effect - and then when you do disclose, she could feel hurt and very confused.
You're not doing anything wrong by not telling her. You're not lying. You should be able to present as you want to present. But the reality is that young people have to be handled with care (and if she's 2 years younger than a guy whose voice could plausibly not have dropped, she's got to be really young). They don't tend to have a lot of experience handling complicated relationship situations, unusual body configurations, rare medical conditions, or marginalized groups. They likely haven't thought a whole lot about gender or sexual identity issues, especially if they identified as straight early on without any soul-searching.
Just keep all that in mind, and make your decision based on what you're comfortable with. If you want to keep things light, fun, short-term, and mostly non-contact, then you definitely don't have to disclose, and having a positive memory of an experience where your gender and sex weren't questioned might be good. But if you see it getting more involved than that, both for the sake of not getting your own feelings hurt and for her sake, you should probably figure out a way to disclose soon. And try to think about the sorts of questions that could come up before you do it, so you don't get blindsided by something (like her thinking that your gender calls her sexual orientation into question, or something like that).
