"Natural" means something which occurs in nature, i.e. as a matter of course, so yes - those things are "natural".
But nobody is making the claim that everything natural must be good or desirable. Always keep in mind that human value judgements don't translate to animal acts.
Rather than simply saying "gayness is natural so it must be a good thing"; they are showing that homosexuality occurs in nature without the influence of human social structures.
The reason for doing this is that it contradicts the vile claim that discrimination against homosexuals can be tolerated because they are "unnatural", or that they are just being 'difficult' and choosing to be gay.