Some people can not bond. They grew up in a dysfunctional family where their parent/s had them on the fringes. Where their insecurity overwhelmed trust. A childhood where the only person they had was them self.
As children they changed their friends like another does their underwear afraid to invest too much in any one person.
Can you expect this person to marry and be faithful?
A person like this can profess love for you, sleep with someone else, and not see the contradiction.
Hence, when you meet someone and they change their friends often, when there are no old friends; unless they just moved to the area, you need to determine if this is a person who can not bond.
If you still think you’ll marry them, don’t expect them to be faithful, for it is not in their nature.