I envy you, being on the brink of something so amazing!
Yes, I agree that if you like a thermometer in there, you might like other things too. But for some people, like me, those other things can only be so large. I've had these in my ass- thermometer, q-tip, finger/fingers, buttplug, vibrating dildo, strapon dildo, carrot, candle, hairbrush handle, and in a doctors office: anoscope, sigmoidscope. Doctors office aside, I loved having those different women put all those things in my anus.
Basically, just take it slow. Transition from large to small, slowly, providing your girlfriend with feedback and hopefully she responds appropriately. It might take days or weeks to build up to a proper strapon fucking. Personally, the fact that she questions your "manhood" is, in my opinion, ridiculous. What the heck is manhood anyway? Just do what you want to do and hopefully your partner is supportive and wants you to be satisfied, and that you want the same for her. I do agree with her, letting her fuck your ass would help you understand what she feels when you fuck her ass.
The woman who used to fuck my ass tried to get me to take a larger dildo over time. At one point we tried the new one, after quite a while of her using her fingers and smaller buttplugs to get my asshole ready, but after a long time trying, the large dildo just was too painful. So we just used the smaller size and continued our assfucking that way. It was still great- the size doesn't matter so much as the fact that I was bent over being fucked by a beautiful woman.
Good luck.