Hm... well the thing is, you shouldn't change. Just because when you change, you are bringing something in that seems like lies. If one day you start getting cute after 5 months of confidence and masculinity, then you have a problem.
I don't believe it is acting macho. Before you can get a girl, you should instill confidence and a masculine side to yourself even before you start dating. The side that you were born with, that nature gave you.
Thing is, the reason why I dissagree is because you aren't trying to show her that you don't care for the first 5 months, or anytime in a relationship period. It's just that you don't do it through words. You should be the gentlemen (ie: open car doors, pull out chairs, walk on the outside of the curb), but you don't have to get mushy and always tell her how much you love her. Love and feelings should all be implied by what you do. If she clung onto your arm and cared for you whenever you needed something, she doesn't have to tell you that she cares for you, you'd know.
There are definite changes as relationships progress. But you shouldn't be the one in control of them. From Day One to Day Marriage, you should still be your confident, masculine self. That is the whole reason she became attracted to you. Of course, if it's all an act, then quit while you're ahead and try some self improvement.
This is just my take on it. If you want to keep things going in the long run, be the man she can look up to and respect. Be the confident masculine man she fell for. Don't tell her you love her. Show her you love her. The words "I love you" should keep their special meaning throughout a relationship. If you say it often, it loses meaning. I don't care what you say about, if she loved you too it doesn't matter. It does. If a woman constantly told you that she loved you, at a time when she really wanted to express it just through words, she would have a difficult time.
Save the "I love you"'s for when you kill her cat or accidentally break her heart and didn't mean to. It'll get you out of a lot of trouble.