If you are asking if someone else can fuck the girls you are dating then I need to answer the girls will be loyal to you. There will be some exceptions, for example Sharon will date multiple partners at once as her approach to cuckold/boyfriends is very fluid, but she will tell you from the beginning how she feels.
But generally speaking you will decide how you want to play it, and for example if you will break up with the girl or reach out a point where threesome could happen, that's the edge result of your actions so simply said it will be also dependent on your choices and if our followers vote for such content in poll votes.