It's been a while since I looked into the research, but I thought at last count, there were well over 1500 species that exhibit at least some level of same sex sex and as for relationships? Those species with a higher order of intelligence show a tendency to monogamous relationships, with some pair bonding for life.
Dolphins, Humans, big cats (lions, etc), most primates.
Yeah, the numbers vary according to different sources. It depends on what each study classifies as 'some form of homosexual interaction.' The most consistent figure I've come across is around the 1500 mark for confirmed homosexual behaviors, but I'm always happy to stand corrected :). Do you have any links to the studies quoting higher numbers? I'd be keen to have a look.
I can check for some recent studies.