ok i'll start off with that some girls DO need larger penises. An ex or 2 i've been with have always gone off, and tried to call me up for some "side action" cause they guys they were currently with weren't doing it for them. When i asked why they were still with them their response (although watered down) was that the guys were nice to them, and it felt comfortable.
If she really loved you then she wouldn't be doing this to you. This is a women looking for excuses to finding sexual pleasure outside of her relationship.
However if by chance you and her are into cuckolding or are looking into the chance of like the idea of a cuckolding relationship (which is basically what you are asking/ confused about emotionally) Then you need to make sure that you EQUALLY enjoy it as much as her. If you simply just WANT to enjoy it because it makes her happy, and you are forcing yourself, then it will NOT work for you.
However if you are interested in an open relationship where both of you just want to receive sexual pleasure elsewhere. Then that needs to be discussed properly. And in all cases of open relationships both sides need to be able to see different people, not just one side.
First and formost you need to find out these 2 things
1.) what you REALLY enjoy sexually and emotionally
2.) and where you really stand for her.
Don't just be the 'safe' guy that this girl wants to be with cause she likes the idea of being in a relationship with you... but doesn't like the sex with.
I'm done rambling now lol