I think it's a "Macho" thing. He should've been able to protect her, it happened on his watch, etc, etc. From personal experience, it seems to me that actually is quite a common type of reaction from certain personality types, when something bad happens to their lover or child, it becomes all about them, and their guilt, rather than about the actual victim.
True enough. That's reason Maggie slapped his hand away: Glen was butthurt over the idea of a sexual threat to "his woman," rather than a threat to Maggie. He has too way too many manhood/property issues, when he needs to realize he's not from a background of violence (ex. Rick, Shane, Daryl, et al) or strong male identity.
Some people with a soft past (ex. Andrea) were hardened by experience, but no one ever promised Glen he was due his change into a badass/street warrior type. Glen seems pissy--as though he's being prevented from manhood, so he lashes out at everyone for no good reason.
Maybe another Merle ass-beating will straighten him out...