If they made possessing bb guns illegal, no one could possess bb guns, and this would not be because of any distinguishing characteristic of the citizen. Whereas, if marriage is made illegal, but only for homosexuals wanting to marry each other, then there is obviously a distinct group who can't marry and a distinct group who can.

So really, the 2nd Amendment is the only part of the constitution that protects gun ownership for everyone. The Equal Protection clause would only come into play if a certain group were specially protected or excluded. (And even then, it wouldn't necessarily be unconstitutional. For instance, if police were allowed to own bb guns, but no one else were, such a law would likely survive an Equal Protection challenge, because it would be judged via rational basis, not heightened scrutiny. It would only fall to a 2nd Amendment challenge.)