What does science say about homosexuality ?

Good morning,

It is said in some of our school textbooks but especially academic dictionary (in France) that heterosexuality is THE natural sexuality. I would like to know what science says on this side because it seems to me that homosexuality is also natural and was not invented like that. I'm speaking for mammals only (I know there are special fertilizations like parthenogenesis in other animals). Or more broadly what science says about attractions (homosexuality/pansexuality or asexuality) and expression of sex (transexuality and intersexuality). I would really like to have some insight into the issue so that I can debate it.