I reckon she'd been sleeping around and made up a bullshit story to fool Joseph into believing she was still a virgin when she got pregnant. And he was stupid enough to believe it so he deserves another man's kid 
Dirty slut
HEY, it was gods fault, to begin with. Can you blame Mary for using it as a crutch to save her ass when all looked bleak. Besides Joseph was a fucking low life carpenter, drinking, carousing and hanging out with disreputable bums. Seriously, why else do you think Joseph disappears from the bible as soon as his part is done. She definitely got an upgrade, respectability, immortality and all the perks that came with being the mother of the "Messiah".
Nice job, if you can get it.

Especially for a woman in extremely patriarchal times! She certainly wasn't nobodies fool!

What you call a slut, I call cunning or cagey.