Just use a lot of lub and relax.
Don't you have a lifesize rubber stiffy like most girls? If not; get one, then tell him you want to practice on him first and shove it up his back door and if he has a problem. That should about be the end the whole subject.
Not a girl, but when I was 18 this much older guy showed me how good it was. He used some KY jelly, and first he fingerd me, one finger, then two, then his dick, it never did hurt, of course he didn't go at it like a stud mating a mare. He took his time and it was great, he said I would be calling him for more, and he was right. I would assume it would be the same for a girl, patience and KY.
My nuts are soo small their hard to even find, But small as they are, its enough male something or other that I won't let another guy even get close to me with his dick. You must have been born without any nuts if you let another guy stick his dick in your ass.
worry some more. i tried it and poop got all over this dick. and my hole didn't shrink right away, which made my have to go crap all the time.
if you don't want to do it, don't. if he makes you it's abusive and if you give in it'll make him think he can do whatever he wants to you