Normal reaction for a gf toward her bf. She expects you to wish to be enough for her and able to make her happy. She may think you want that reaction from her, telling you how good you are and you make her feel. She asks you why you keep saying "this". And how do you respond? Do you tell her exactly why you do? And what`s on your mind, for her to have the "BBC" experience? How will she know if you don`t tell her?! (And also explain to her WHY you want it so bad!)
If YOU are real and it is YOUR goal to have your girlfriend "blacked" then it is on YOU to take steps that it will actually happen. You can`t just sit and wait what your woman will do next. She won`t!
Telling her your "cock is kinda small" will not make her lust for a bigger one but maybe feel pity for you and get frustrated. It`s no good to actively make your own girlfriend feel that way - it`s bad and sad.
As far as i read @muscuck she fucks HIM already and thay are NOT married. From the writing i suspect both not to have a high age, and AFAIK most muslims that age in Germany were living there most if not all of their lives. So i guess, as far as culture is concerned his gf is not "out of this world".
But it`s true, strict muslim women are not to have sex before marriage, only "allowed" to marry muslim men, and marital infidelity is a big crime for both men and women. If she was brought up this way it would probably be a hard task to "convince" her to have sex with other men on the side.
Well, however, she is a woman , she already fucks without being married, and, (my assumption ->) a non-muslim man, so chances live...
Women are to marry muslim men only (those who already are or those who convert) and are only "allowed" to fuck when married, men are "allowed" to marry multiple wives no matter their religious belief, at the same time to raise children according to the father`s religion is a must. Rules made in order to help spread "the own" religion.
It is comparable with the christian set of rules to raise children according to the christian belief, when at the same time it is a "law" to "honor one`s parents" (= not to question the way they bring you up) and the prohibition of use of contraceptives and "alternative" sex practices like anal. -> Also helps grow "the own" religion.