it doesn’t matter that peggy and steve had a happy life when steve travelled back in time. it matters that steve canonically knew that peggy had lead a good life, moved on - got married and had kids and grandkids and IGNORED all of that in favour of HIS happiness. he had no way of knowing that peggy would be happy with him. their relationship consisted of war-time pressure and (1) kiss. realistically, a lot of relationships formed under high pressure situations like WAR fall apart once they’re moved to civilian circumstances. yes peggy had a photo of steve on her desk, but it is in fact possible to love someone but not actually want a relationship with them. a lot of people like to leave the past in the past, so they can preserve the good of the romance.
steve chose his happiness over peggy’s because he decided that him wanting peggy was more important than the entire life she created after he died - and thats why its fucked up