Why men's love decrease after sexual intercourse?

Sure. If this is true, then the reason most likely is that these men have been taught that sex is something dirty and that women with whom they have sex are made impure or less valuable by the act. I do not see this as something natural but based on indoctrination. The way to test this is to see the extent to which this is true in cultures that stigmatize female sexuality (such as Muslim and Catholic) as opposed to cultures like Native American that do not.

Sure. If this is true, then the reason most likely is that these men have been taught that sex is something dirty and that women with whom they have sex are made impure or less valuable by the act. I do not see this as something natural but based on indoctrination. The way to test this is to see the extent to which this is true in cultures that stigmatize female sexuality (such as Muslim and Catholic) as opposed to cultures like Native American that do not.

not true.  The reason why men are tired after sex is because they lose vital nutrients after ejaculating, unlike women.  Also, to explain why you are wrong, evolutionarily-speaking, women cheap as much as men because men, in the cave-men era, were more likely to die un-expected due to the dangers of hunting, so women constantly seek a man on the side in case that happens.  Men also form just as strong of permanent bonds with women, because a child grows up more successful with two parents.  One more thing I should add:  To look at the cave-man era and think men had the opportunity to "spread their seed" does not look at the facts of social life during that era.  People lived in very small communities where everyone knew everyone else, and by looking at primitive tribes of today, it is not likely there were many men who "spread their seed" by having sex with multiple women.  That is more of a modern phenomenon caused by men in large populations where no one really knows each other, which is un-natural.