What do you consider a relevant link?

[Jace]

I know what I consider to be a relevant link, and it has done me well over the years...basically, I don't trade links with a mobile phone site if I am running a hardcore porn site

but when we get down to specific niches, does it really matter...like trading links between a anal site and blowjob site....does google really see that as non-relevant? I know for US they aren't in the same niche and that blowjob traffic won't be relevant for anal traffic, but are SE's seeing that as irrelevant linking?