Not at all.
I see a lot of guys my age that are with one girl one day, then the next the next day ... in short, they change them more often than underwear. This sort of behavior is a turn-off for MOST girls (at least the decent ones). Not only will they have no trust for you, but you will seem insecure, flimsy, undecisive and superficial. Well, that, my friend, is the exact opposite of being masculine.
I also see several guys who are in good, stable relationship and act well towards their girlfriend. They're protective of them, seem happy and loving. They have also decided on ONE girl, which makes me think they must have strong and determined characters, and definitely know what they want. Such guys I see as far more masculine.
The myth that men who hook up with a lot of women are "studs" might have been true once. But it looks pretty idiotic these days - this is also true for women who go and sleep with a new guy every day.
It says a lot about the person ... and it isn't good things.
So if you want to be in a serious, long-lasting relationship, go for it. :) Most girls would feel respect for a guy like you.