"No" doesn't mean "no" when the girl is laughing and smiling and teasing you.
If she says it in a serious tone, then take it as "no"
You can tell when she's just saying it to get attention or tease you, or she really means it- it's pretty easy.
As far as the first few times you're with someone, yeah, you should err on the side of caution and take it as a "no"...
My husband has a pretty low emotional intelligence IQ, but he has no problem at all differentiating between my fake "no"s and real "no"s.
And I hate scewing with the lights on- dim is okay, but I don't want my husband staring at my ass while he's behind me if I feel that I'm having a bad ass week- I get a dimple on it when I retain water from pms.
The only times I have ever been able to screw with all the lights on and stare in someone's eyes and act like an actress in a porn flick, were with cheap, shallow sex only very short "relationships" when I knew it meant nothing and I didn't care how the guy felt about me.
So, the fact that your wife is self conscious means she loves you and cares about what you think
