Few guys would ever admit to being intimidated by another man. A man likes to think of himself as a "real man", a "man's man", a "Viking" who is master of his domain. Men are conditioned to be strong, fearless providers and protectors. They are wild bucks, kings of the jungle, and conquering Caesars who back down to no one, until finding themselves in the presence of the dreaded ex-boyfriend.
Men are uncomfortable with the thought that the chastity of their love has already been violated. They may push the thought aside, or accept it as a reality of life, but they would rather not ever be faced with it. A man may be confidently self assured of his place in a woman's life in front of men with whom she has no interest, but just the thought of her being with a flame from her past can make him doubt his relationship.
Why are men so intimidated by ex-boyfriends? Why is it that the testosterone kicks in with just the mention of a name? One reason is because a guy realizes that there will always be someone else who was special to his woman. He may be a girl's first "real love", but he could never be her first love. Regardless of how secure he may be in his relationship, he always knows that the "ex" had her heart before him. Most men want to see themselves as the only man to truly rock a woman's world.
There will also be the question of what feelings remain between a woman and her "ex". And some women can cast further doubt with statements like, "I still care for him, but I am no longer in love with him", or "He's in my past, I'm with you now". Though reassuring and momentarily comforting, those comments do not erase all of a man's doubts. As much as he may try to convince himself, there is always the thought that she may still care for her former boyfriend more than she admits.
There are other reasons why a man may feel threatened by a woman's past. Men can be territorial, egotistical, and internally emotional. If a woman frequently references a previous boyfriend, or maintains some level of close friendship with him, then a man may feel that his territorial rights have been violated, and his ego turns that into a "respect" issue. He may express his feelings through rational dialogue, but deep inside, he is emotionally crushed.
Some women use their relationship with their "ex" as an item of negotiation. They dangle their association as if she could always go back. For some it's a game, but for others, it becomes a serious threat when things are not going their way in the relationship. It is a form of manipulation that may keep a man forever wary. Men often want to create fond memories in a relationship, but they never want to compete with them.