I chose birds because it's a taxa I know well and I knew to be largely monogamous. I've shown 90% of birds are socially monogamous, and 1/6 of vertebrate species are birds, so at least 15% of all vertebrates are monogamous, and that's plenty enough for me. If you want more exact numbers feel free to look for them yourselves, but I think I've given you a pretty good starting place. For the record, I doubt you'll find anything in the existing literature which attempts to give exact percentages, mostly because it's a rather useless figure. Taking averages over species assigns equal weight to some obscure insect species on one particular island with only a couple hundred individuals to a species with a global distribution and billions of individuals. And even once you get that number, the average sexual behaviors across all species doesn't give you much insight.