.......they'd change
1. they ignore boys nights out. Going out with a girlfriend apparently is the most fun a boyfriend can do on weekend. Not just about having the pleasure in that boyish way, but also letting out the real emotion to the opposite sex who'd respond to their inner sensitivity with good hugs and feelings, instead of a punch in the shoulder saying "oh, you can go thru this shit, man!".
2. they spend more money for cheesy thing. Flowers, chocolate, cards, surprises and gift voucher. Making the girlfriend happy is always a self-satisfactory. Not only expecting something in return per se, but also seeing the loved ones smiling and happy is an achievement too.
3. they become drama kings. They can have a mood swing and become an insecure bitch just like girls who're having their PMS. Guys can be so envious and highly tempered when it comes to girlfriend thing. Most of them can put everything under control, but some will always show affection, possession, vulnerability and emotions deliberately, in public.
4. they improve their self-esteem. No matter what. Having a significant one makes a guy feel more confidence because they're trapped in mind that there is always this one particular person who'd accept and love them for what they are. They can go chin up amongst all the single (or not) other guys and implicitly shout "I've got this hot loving girl with me, dudes!" to the world. Or.........
5. they turn out to be a jerk. If one can get a girlfriend easily, then getting a two or three must be really adventurous and challenging. How? It's the nature of boys to keep answering the 'how' meanwhile women are busy finding the 'why'. Besides, 'how' is just as much explorable as 'why' but in a bit manly manner.