From a polyamorous perspective. I had that happen once when I was 40. Wife was OK with relationship with the 20 year old woman, 60 year old girlfriend was ok with it, but the 25 year old girlfriend was not. She didn't trust her. I ended up breaking up with the 25 year old, but turned out she was a better partner for me than the 20 year old (the years did matter as far as non-sexual connections).
I never spoke about age really, but realistic not many girls would like if one is
(unfaithful). But I never said there can't be a harem or multiple girls-guy option either.
It has happened in our universe-though it is way less common these days.
When I say Unfaithful in a relationship I mean...
Being Unfaithful is foremost in my opinion lying,
to the partner of who they were/weren't with and such...
Say A guy/girl promises their partner to be faithful and then
next week decides to flirt and have sex with a hot random person?
-while of course not being on a high drug or something...
Or instead of a random person it could even be a hot friend or hot EX-boyfriend/girlfriend
or something like a family friend the person finds hot/sexy?
But it could also be something like breaking a special agreement behind
his/her partner back This while also of course being of 'sound mind and body*
Say Felix is allowed by Felicia to have many girls except 1 girl Named Anna.
say Mary has been mistreated by or is a daughter of a rival to her father's
business or something? And yet Felix still goes and pursues Anna
during say a party? 'even though there are other girls there he runs after
Anna Even though he promised Mary that he would specifically not do that.