I think your gf is to young for the pleasure of such "hardcore" games. And as woman with nearly 46 years I can tell you: You do not want a "slut"! You want to see(!) a "slut" and after that you want your gf back. What will you do, as she will be catched by a big dick? She is young and perhaps you open a door, she will kick you through beacuse she learns a new way of sex? Big dicks are very hot - for young and older women!
In the age of 20 sex was for me nice and I had a lot of Lovers, but the sex was not nearly the sex I experienced from the time on my husband said the "golden words" to me. Give her the time she needs and do not ******* her.
At the end: We mature sluts will give our husbands the better and depper performance in case of cuckolding. ^^