How can understanding the justifications people use for cheating help in preventing or addressing infidelity in relationships?

Why when things start to get romantic between and guy and a girl, one of them blocks the other one? This is a pattern I noticed when it comes to having somewhat of a relationship with the opposite gender and things start to get romantic.
What should I do if my significant other’s family doesn't invite me to family events like weddings?
Where do you think the biggest pressure on men comes from?