I have to disagree strongly with this statement. (Nothing personal to you Shikubokubo. It's just that I've heard this so so so often and find it stereotypical at best.) As someone who's been married twice and who has been in several serious relationships, I'd have to say that, in private moments, a lot of guys actually do talk like that. It's a given that most won't be lining up in the isles to publicly shout their emotions to the world. However, in my experience, most men don't have an issue with spouting poetic and romantic things to someone that they have deep emotions for. Not to say that I haven't known many men who were incapable of this feat.
In those cases, it is generally a case of insecurity covered by a front of 'macho man attitude'. They're like 'Look at me. I'm a man. A manly man. I have no emotions because I'm so incredibly manly that I make robots look like emotionally driven pussies. Now bow down and admire me in all of my manly glory!' Okay. You get my point. Men are human. Period. They have all of the same emotions as females do. They get angry, sad, happy, whiny, lovesick, angsty, and sappy. It's part of the human condition.
How they show these emotions tends be to a reflection of their upbringing. If they are raised in a home where the people are demonstrative and outgoing with their emotions, then they tend to do that as well. If they are raised to believe that 'real men' don't show emotions, then the likelihood that they will be the type to shower their partner with pretty words is less likely unless something dramatic happens. People who have experienced a lot of loss in their lives also tend to be more demonstrative (or less depending on whether or not they decide to protect themselves from future loss) than most.
I think that whether a character is the type that spouts flowery, poetic love declarations or goes into long, dramatic explanations of their feelings shouldn't be determined by what sex they are. Because, in real life, it just doesn't always work that way. Case in point, I find that I'm not the type to spout my undying love for my partner. It makes me feel queasy. Hugging people (even close relations) makes me feel awkward and uncomfortable. Does that make me a man? Not particularly, and I have the equipment and a couple of kids to prove it.
Whether a character is emotionally/physically demonstrative should be determined by the how the author interprets the character based on the background, upbringing, and personality that the author has developed for that particular character. If a character is lonely or alone in the world, and desperate for some close connection to another, it would be perfectly acceptable (in my humble opinion) if that character started declaring their undying love for the one who'd made their life complete. On the other hand, if the character is a bad ass biker who lives his life on the edge and avoids emotions at all costs, even if this person did find the one they want to devote themselves to, they're not very likely to go into a long, detailed diatribe about their feelings on the subject. I guess my point is that this is an individual thing and should be determined by the individual character.
Bear in mind that I'm referring to mature, adult men here. This will most likely not apply to a teenage boy who has made it his life's mission to prove his manliness and who turns the practice of machismo into a true art form. Just thought I'd throw that out there for clarity's sake.