I'd personally find it a little odd if a guy asked if he could keep his shirt on. If you're not comfortable enough with someone to be completley naked with them then I'd boldly suggest that perhaps you shouldn't be having sex with them in the first place.

I was afraid of that. Thanks.

Just turn off the light.

Plus she will be focusing on sex while your having her your back scars wouldnt even cross her mind.

Wait 'till you get a gf......