I think what you're really asking here is if it makes you less masculine.
In my opinion...yes, probably so. Not necessarily all of the time though.
Is that a bad thing? No, absolutely not.
Does that make you more feminine? No, I don't think so. I suppose you can incorporate femininity if you like frilly dresses during play or something though.
Does it make you childish, boyish? Well, yes, that's very much involved in the thought process.
It just doesn't make you a big, burly lumber jack that opens beer bottles with his teeth 24/7 type. Not that you can't sometimes be that and sometimes be a little boy though.
How often does this lady want to to be "Mommy" with you? You need to ask that because it can make a huge difference in your relationship structure. Perhaps she expects you to generally be masculine and do "manly things" or take on normal adult responsibility during the day but after the daily work, chores, and adult needs are taken care of she wants you to regress, submit, and need her. Perhaps she expects you to regress very often or very sparingly. We don't know that.
You can still be a "brave little boy" for your Mommy too though. Just because you're acting childishly doesn't mean that sometimes you can't do big boy, adult things. I mean, at the end of the day, if you two were out and your Mommy was being physically assaulted by some stranger then identifying as a "little boy" doesn't mean you can't act like an adult (or even "manly") and step up to protect her, go get help, or scare the stranger off through physical actions. You probably wouldn't just stand there and sob, weeping like a child might actually do--and, even if you did, it would probably be out of pure shock and terror instead of being incapable to make rational, adult decisions on your needed actions.
You're not being stripped of having the ability to do manly things either. You have a choice. You are an adult with adult mental capabilities. You know how to be a child but you are also an adult.
As an adult in today's world, I think the very least of your worries should be, "Am I being manly enough?" If this lady likes you, and you like her, then do what you two enjoy together. Forget the stereotypes and the outsiders. You just be yourself and let her enjoy those moments with you.
You are special. She is special. What you two have together is special. It's not anybody else's business as long as you two are happy and not hurting anyone.
Don't be afraid to enjoy life.