Because it's our biological nature.
To invite someone into your bed is to tear down a wall, a wall that we as men need to keep our animal nature in check and our hands off you.
It's a gesture of intimacy, or even just trust when a woman tells you it's OK to share a bed. She's letting down her emotional defenses: 'the bubble' if you could call it that. There's a perfectly acceptable couch in the other room, to invite a guy into bed and expect them to behave is... odd.
Heck, that's more closeness than most relationships or hook-ups start from.