The default value for Unsubscribed is False. Marketo doesn't require you to prove that people opted in to emails. However, I'm pretty sure it's in their TOS that you won't email people who haven't opted in.
Reading between the lines, it sounds like you're interested in emailing a cold third-party list. Let me caution you against that. The quality of those lists is usually anywhere from so-so to god-awful. If you end up with a lot of bounces, you're going to wreck your sender reputation. You're also putting a bunch of junk in your Marketo instance. You could also get caught in a spam trap (which may cause Marketo to suspend you from sending emails until you fix it, it's really not fun). If you decide do to it, at least run the list through something like BriteVerify first.