A Nice Guy is a man that typically has very low Alpha Traits, and a good amount of Beta Traits. This means he can establish what amounts to good friendships with women, but cannot convert those friendships into the sexual relationship with them that he wants. He’s always confined to the Friendzone with them, and watches helplessly in pain and confusion as “less nice” men make easier progress to having sex with his love interest.
Usually at the start of a serious relationship, the man has both Alpha and Beta Traits working for him. Once married though, there is usually a persistent pressure on him from his wife and “helpful advice”, to become increasingly Beta and do less and less of the Alpha Trait things that initially attracted his wife to him. This is called Betaization and the result is that the wife slowly falls out of love with her husband, and the husband turns into a Nice Guy inside the marriage. Typically the sex dies away to a slow trickle when this happens.
Importantly, this process happens almost automatically and very frequently the wife makes strong demands that the husband comply with being more Beta, less Alpha. The wife is never aware that her attraction for him will diminish when he complies with her.
It’s really just as simple as when they were dating, he rode a motorbike, went rock climbing and worked out. But now he drives a mini-van, watches sports on TV and doesn’t have time to work out anymore because he’s got a proper job now.