Any guy who pushes your head down and assumes that eye to penis=mouth to penis needs to be shot. You want her to use her mouth? Use yours: ask. Keep in mind not all girls are comfortable with it, so she may say no. She may need a little more coaxing if she's shy, but by no means force her to do anything she doesn't want.
I suggest you go down on her first. This gives you leverage because if she won't after you do she's being a b1tch; if you don't go down on her and expect her to go down on you then it's you who is at fault because you're being unfair. Get a woman off in bed she'll pay you back ten fold, get a man off, he'll leave. Women know this, and several have a rule that they won't go down on a guy unless they're reciprocating specifically for that reason.
That being said, it is not an unspoken rule so you're right, you still have to ask.
How to ask? Two methods. One, talk to her OUTSIDE of the bedroom. People get stage-fright in the heat of the moment and this way if she has any apprehensions she can discuss them without worrying that she's killing the mood. This method works best for ALL types of sexual activity.
And unless she just lies there, she probably kisses you on more places than your face. (Or at least let's hope so.) Moan louder when she gets nearer to where you want to be kissed, kind of like in kidergarten when you played the "getting warmer" game. The downside is she may not pick up on the hint, in which case refer back to the above method.
