If, as you say, the person honestly wants to participate, the answer is yes. People who are sincere about wanting to participate in any topic can do so.
Two little side-bar notes to this:
1) Sometimes what seems like an honest question to an outsider comes off as rude or condescending to people inside a group. Like, if you were to enter the vegan thread and say something like "Isn't all vegan food garbage? Also who gives a shit about animals?", there's basically no way people would interpret that as an honest question from someone trying to learn. Instead, it's likely people would react to you like you're trolling. And maybe you were being honest, but it's going to come off like you lack tact. A very common tactic that trolls use is playing coy or naive about their true level of knowledge or intentions with respect to the topic they're discussing. You might not be a troll, but if you seem like one to everyone else, you're going to have the same effect a troll would.
2) In general, the thread title should match the thread contents. So if a thread for girls on GAF is 95% guys trying to ask questions of girls, it would be reasonable that the girls might say "yo, we're not here as your lab rats". Not that guys can't post in said thread, but that if they're overwhelming the intended community for the thread, then the entire thread is sort of worthless. The same could be true for an interest-based community if most of the respondents were outsiders asking basic questions. So respect is about recognizing that you can participate in a thread without making the thread about you.
I've seen non Brits get shooed out of BritGAF, so I'd imagine for some communities it's more aimed towards specific users, which is fair enough.
I think much more context would be needed there. If a non-Brit is monopolizing the discussion by using the thread as a petting zoo, or by drowning out other voices, or by preventing discussion from occurring, then thats disruptive. But it's not as though someone shouldn't be able to weigh in on a political question just because they're not a citizen or whatever. Of course it'd be reasonable for others to point out that as an outsider, their opinions might have less weight within the community than insiders in certain cases. Reasonability should be the thing governing like 99.99999% of interactions.