Whilst not romantic love, other primates (gorillas for example) have a single dominant male who tend to a covy of females. Wolves and mountain sheep do it. This behavior has been assumed to be a method of optimizing the species strength by assuring the strongest males' genes get passed along. Maybe they are just smarter.

I know I can love more than one woman at a time. I think (in the US) our christian dominated moral directives supress love. I'm not sure why.