It will be unpopular to say this to anyone who is still pretty young and has had a family with a man they've been with since their teens, but the simple fact is that one or other of a couple is far more likely to get itchy feet and start feeling they've missed out on their 'sowing wild oats' years, if they have only had one long term relationship from a pretty young age. I think either the man or the woman can feel this way, but the truth is that the man is more likely to act on it, because he:
a) often gets more opportunity to be outside the home and away from the day to day care of the children
b) will often (rightly or wrongly) feel a bit aggrieved at being 'replaced' in her affections/attention by the children and will bemoan a lack of sex, whereas women are so preoccupied with the physical and emotional demands of childcare that sex becomes temporarily very much secondary.
c) is a man, and whatever feminists say about men and women being essentially the same, and everything else being a social construct, generally speaking, as a man he is not genetically/hormonally programmed to mature, or to take to monogamy as easily, or as young as most women.
You only have to look at the animal kingdom to see that the less evolved/civilised a society is, (or for humans let's say 'industrialised and developed) as it sounds less contentious) the more misogynistically it is set up. The males quite literally leave the woman 'holding the baby' and having no qualms about serving their own interests above everything else. In some parts of the developing world, routinely impregnating a woman (or several women) and then feeling no responsibility towards them whatsoever is so commmonplace as to have none of the societal pressure or guilt attached.
Of course anyone of any age can have an affair, or just bail on the responsibility of marriage and children, but there is no getting away from the fact that it is more likely to happen if a man has felt 'tied down' too young.
I know someone will come along to argue with paragraph four but I don't care. 