Couldn't be further from the truth. Honestly I think its the other way around. Women are the ones more guilty of trying to chance their partners.
Men are usually more privy to the fact that once a person becomes an adult, they're pretty much set in their ways and will not change. Smart men will have this idea in his head of what his ideal mate will be like. That incorporates everything. Looks, style, interests, morals, values, career, family values, etc.
When we meat a new girl, obviously (as with women), our first point on the list is looks. Partially because at the beginning its all we have to go on. We don't know you yet. Plus nobody wants to date someone they don't find attractive. That being said, guys are actually proven to be less picky with looks. Guys are generally willing to date a girl who isn't a perfect 10 if she has a great personality. If we find her attractive, then the next step is to run down our mental checklist, and see how close a match she is to our ideal partner. We'll literally create a mental list of the pros and cons about the particular girl. If we feel she checks out well in the majority of the categories, then we'll likely want to date her. If she doesn't, then instead of trying to change her (like women do), we just find someone else.
I think where the confusion can come in though is some guys instead of moving on to someone else, will just say "well this girl isn't worthy of anything serious or marriage, but she's at least worthy of sex since she's hot". So he'll have this little fling just to get laid until something he really wants comes along. The entire time, girls like you were completely unaware that he never really liked you to begin with and was just using you for sex. For guys, this is totally obvious. From the outside looking in, we can tell what the guy is doing from day one, but for some reason the girls just can't pick up on it. The guy will clearly be using them for sex and the girls seem to think he's madly in love with her.