It makes the man feel like he is totally accepted and desired by the woman is the biggest part of it.
It's hard to explain the feeling it gives me but when she does that for me it's just fabulous!. Men are "very visual" as well, and to watch her do it is a HUGE part of the thrill. I can see it replay in my head over and over and over for days afterward!
So, if you really have deep feelings for your guy and want to demonstrate it, this is one of the best things you can do to show him. It's not a guarantee but it will make it MUCH less likely that he will stray as well. A man becomes very content when he's with a woman that will swallow and completely accept him. No man wants to give up that kind of bond!
Follow your instincts and if you can find it in yourself to do this for him then do it. Men LOVE it when their girlfriend/wife swallows. It's hard to explain but it's something about total acceptance in a man's mind and EXTREMELY erotic.
Like I said, men are very very very very visual and seeing a woman do that for them is a big part of it. That's why porn is targeted primarily at men and 99% of the time a scene ends with showing the man ejaculate onto the woman and her writhing in enjoyment. They do this because it's what many men dream of experiencing and want to see in their own lives.
The woman that will accept their seed onto her tongue, swallow and smile, is an absolute dream come true to most men!!!!