〜たち is a suffix which means more or less "the group to which <someone> belongs". So ぼくたち refers to the speaker as ぼく, but it also refers to the group to which they belong, and it can be used as long as ぼく is appropriate for that one person.
In other words, it's not a "plural" marker, so it doesn't matter whether the group is mixed-gender or not. They're not all being identified as ぼく. Only one person is.
Of course, the same thing is true if you attach 〜たち to a name. If I write 田中{たなか}さんたち, I'm not referring to a bunch of 田中さんs. I'm referring to one person as 田中さん, and I'm also referring to the group to which 田中さん belongs.
And finally, the other similar suffixes in Japanese, such as 〜ら and 〜がた, function the same way. 彼{かれ}ら refers to 彼, but also to the group to which that person belongs. So it can be used for mixed-gender groups too, as long as it's okay referring to the person in question as 彼. And so on.