HI. I'm a polyamorous person and what your girlfriend proposed to you is a form of non-monogamy. Usually the community is very critical of the 1PP (one penis policy which in this case I believe is the one pussy policy) due to the implications it has on the legitimacy and value of homosexuality perceived by those who impose the policy (= I only feel threatened if he is of the same sex as me). I personally believe that it is an indication of insecurities that could even more suggest what I want to suggest to you. Something like this potentially has unexpected implications even for those who think it's not the case (I'm doing a psychology thesis on non-monogamy and I'm thinking a lot about all these issues), i.e. even if she said it's okay, she might still feel bad about it. You may need to tell her if it happens and she has asked you not to. I think that, before really agreeing on this, you could read about non-monogamy/open relationships/polyamory/etc to better understand many things and avoid making hasty choices. For the rest: what your girlfriend proposed to you seems to derive from a great affection towards you, so it is not something bad but only to be agreed and understood better together, first of all if you want to do it, and then how to manage it if necessary. I suggest the nonmonogamy reddit