I suppose if you define Islam by the literal translation, "submission to God" then the argument could be made that Islam is natural. However if you're being at all pragmatic your definition of Islam would have to include the accompanying texts, practices, and dogma associated with Islam. After all, I consider myself in submission to Allah by the sheer fact that as an omnipotent and omniscient being and the creator of the world, Allah exerts significant control over my existence. But no one would consider me a Muslim just by virtue of this "submission". By a more reasonable definition, Islam (and all other religions) cannot be considered "natural". As human beings we are not born with any specific religious practices. We are instead socialized to conform to a certain religion, and anything we do that is considered religious is a learned behavior. This doesn't make religion less powerful, or imply that you can't differentiate between the relative "rightness" of different religions, but it does mean that non of them are natural.
As for the spread of Islam by force. Islam was spread by force in some instances (Moors in Spain), by persuasion from societal benefits in others (Ottoman Empire), and by voluntary conversion in other instances. Contrary to popular belief Islam did not originate as a "Black Man's religion". Even today South Sudanese African are being slaughtered and enslaved by North Sudanese Muslims (Arabs).
Peace


"We live now for the promise of the infinite" Mos Def