This may sound weird, but I've been curious. Can Men have children with Mer? Is there an override? The Gray Prince's father was a vampire Imperial.

Is there any lore on this? And how come Mer can't really reproduce as fast as other races?

Also....what about.... beast races......?

Children are always the race of the mother. Men and Mer can reproduce. It's less known about Khajiit and Argonians but Khajiit can probably interbreed with Men and Mer because they themselves have Merrish origins.

You should read http://www.uesp.net/wiki/Skyrim:Racial_Phylogeny is you haven't already.