Should You Take "Your Opponent's" Sheep In A 2-Player Game?
I've been allowed to cook 4 sheep in a 2er before and still lost the game.  Badly.  I do think in general, your opponents are over-valuing "scattering the woolies" (as one BGGer puts it) if they're just making a knee-jerk response to your First Fireplace.  Assuming you keep one, it's theoretically a 2-point move, but only if you don't cook it later and never take Sheep the rest of the game.  Otherwise you're spending an action (that you would've done anyway later) just to block.  Blocking is necessary in 2ers, but you need to do it in a way that's a net positive in score for you (e.g., if your opponent gets begging card(s) because of it).

I think if you're that worried about your opponent getting 8 food, you're probably already behind.  There are enough ways to get food in a 2er that "stealing" some sheep isn't likely to make up the difference.