Three dogs meet up for the first time, and the group of us are entertained by the behavior that ensues. Amongst more "innocent" play, the dogs display some brief mounting (to establish social order, or perhaps simply as a part of play...). When the male dog attempts to mount the female dog, everyone seems uncomfortable with that and kicks them apart. When the male dog attempts to mount the other male dog, I hear comments like "you're not supposed to do that, don't you know you're both male?" as they are kicked apart. Don't get me started on how ignorant this comment is! :) It assumes sex is only for reproduction, and has no other importance (social, pleasurable, playful, etc) for other animals. And while these people aren't homophobic at a rational level (and for humans), their comments do illustrate their heterocentrism in general.
If you want to know more about social and other uses for sex (in all species), and the innumerable instances of non-heteronormative behavior amongst non-human animals, take a look at Evolution's Rainbow by Joan Roughgarden.
