Basically, a man is "programmed" from childhood to keep his guard up and never look weak or vulnerable. And when you think about it, a man is most vulnerable when he's naked with a hard-on and about to shoot his load.
So when a guy is getting a good blow job, but he's not moaning or squirming about it, then he's just protecting his rep and proving that he's in total control of the situation and his emotions-- that is, no matter how good it feels and how bad he wants to squeal about it!
I've watched a lot amateur military vids with str8/bi guys getting jerked and sucked off. Most of them hardly ever flinch. They compare moaning and squirming to what women do during sex and they don't want to appear feminine-- especially if there's another guy in the room.
Personally, I love loud sex-- with women or men. In fact, there's nothing better than breaking a guy down; making him weak and feeling so good to the point that, at orgasm, he doesn't even care about his rep anymore. He's just an emotional heap and panty waste. A beautiful thing to behold.
Know this one thing: At the moment of orgasm and ectasy, when a real man lets his guard down to moan, squeal or squirm, that means that he's comfortable with you and totally trusts you enough to let you see him in that position. It's a golden moment.